Before you spearfish — this comes first
- Get proper instruction before you spearfish. A recognised freediving or spearfishing course teaches the things that actually kill people in this sport, and reading about them is not the same as being taught them.
- Dive with a buddy, and dive one up, one down — one diver in the water, one on the surface watching them, every dive. A breath-hold blackout is silent and gives no warning; a watching buddy is the difference between an incident and a fatality.
- Stay inside your own training and experience. Depth and breath-hold times are personal, they change day to day, and nobody else's numbers apply to you.
- Understand the conditions before you enter the water: visibility, current, swell, tide, boat traffic and your exit. Conditions decide whether a dive is reasonable far more often than skill does.
- Use equipment you have been shown how to use, check it before every session, and be visible — a marked float and line are how boat traffic knows you are there.
- Never extend a dive for a fish. No catch is worth pushing a breath-hold, and the urge to take one more second is exactly what a blackout feels like from the inside — which is to say, like nothing at all.
Reading underwater conditions
Explainer · Last reviewed 2026-08-10
Visibility, current, swell and thermoclines decide whether a dive happens and what it can achieve — far more reliably than skill does.
Safety
Moving water and currents — Can cause serious injury or death
Current and surge decide whether a dive is reasonable far more often than skill does, and both are frequently invisible from the shore or the boat. A diver who enters in slack water and works down-current can be unable to return against it, and surge near structure moves a diver into rock rather than the other way round.
What reduces it: Assess current and swell before entering rather than after, plan the session so the return leg is the easy one, and set a turn-around point on the surface. Abort early: conditions that are marginal at the start of a session are worse at the end of it, when everybody is tired and cold.
Worse when:
- On spring tides and around headlands, races and narrow channels
- When wind opposes tide, which builds a short steep sea over the same ground
- On a falling light, when the exit becomes harder to find
Shallow water blackout and breath-hold loss of consciousness — Can cause serious injury or death
Poor visibility, cold and current each raise the work a dive costs and shorten the margin, and they arrive together far more often than separately. A diver applying a familiar dive plan to unfamiliar conditions is applying yesterday's margin to today's water.
What reduces it: Treat conditions as a reason to shorten dives and lengthen surface intervals rather than as an obstacle to work through, and keep one up and one down so the ascent is being watched.
Precautions that always apply here
- Never spearfish or freedive alone.
- Dive one up, one down: one diver down, one on the surface actively watching, every single dive.
- Tell somebody where you are going and when you expect to be back.
Conditions decide more about a spearfishing session than technique does, and the four that matter most are worth understanding separately because they behave differently and are read differently.
VISIBILITY governs everything. It sets the range at which fish can be identified and shot, the range at which they see you, and whether an environment is navigable at all. It is degraded by swell stirring the bottom, by run-off after rain, by plankton blooms and by tide state, and it can change completely over a few hundred metres.
CURRENT is the one that ends dives. Working against it burns oxygen at a rate that shortens every breath-hold, and a current that is manageable on the way out is a serious problem on the way back. It also changes with tide state, which means a session planned around slack water is a different session from the same ground two hours later.
SWELL affects the surface, the entry and exit, and — inside kelp or over shallow reef — the water column itself. A diver in swell over shallow ground is being moved around while trying to hold position.
A THERMOCLINE is a sharp boundary between warmer surface water and colder water beneath. It matters for two reasons: crossing it is a sudden thermal load, and visibility frequently changes across it, sometimes improving dramatically.
Deciding not to dive
The most useful skill in reading conditions is the one that produces a decision not to get in, and it is worth stating plainly because the sport's culture does not always reward it.
A dive that is called off costs a wasted journey. A dive that should have been called off costs considerably more, and the mechanism is usually compounding rather than dramatic: the current is a bit strong, so the swim out is tiring, so the breath-holds are shorter, so the diver stays longer to make it worthwhile, so they are colder, and the exit that was marginal on the way in is now being attempted by somebody with nothing left.
None of the individual steps is a decision anybody would defend. Together they are how most incidents happen.
The practical version is a set of conditions agreed in advance, on land, in the abstract — above this swell we do not enter here; if the current means the swim out takes longer than X we turn round; if visibility is below what we can identify a fish in, there is no point. Agreeing them with a buddy makes them harder to talk yourself out of, which is the entire purpose.
Authoritative guidance
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
- BSAC diving safety guidance and annual incident report — British Sub-Aqua Club (Water-safety or diver-training authority)Relied on for: The principle that conditions assessment and pre-agreed limits are part of dive planning rather than in-water judgement.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: The nature of swell, current and cold-water risk for people entering the sea from the shore.FishingHQ's interpretation of what it saysPassage read and verified
Where to go next
Where it matters
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Spearfishing
A membership assertion rather than a topical one: this record teaches the discipline or its equipment, so the discipline's mandatory safety posture applies to it.
- CMAS technical and sporting standards for underwater activities — Confédération Mondiale des Activités Subaquatiques (Standards body)Relied on for: Breath-hold spearfishing as a defined discipline with its own technical, equipment and safety standards, set out by the international underwater federation.States this directlyCitation checked — the work exists and is described correctly
- DAN guidance on freediving safety, hypoxia and shallow-water blackout — Divers Alert Network (Water-safety or diver-training authority)Relied on for: That the safety requirements of breath-hold diving apply across the discipline rather than to particular equipment or techniques within it.States this directlyPassage read and verified
Where this applies
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
The Atlantic coast of Ireland
A link from a place to the physical mechanism that governs its fishing, which is a claim about coastal process or hydrography rather than about angling.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Ecoregion description, stock distribution and the hydrography of the sea areas named, including seasonal inshore movement.States this directlyCitation checked — the work exists and is described correctly
- Cefas science, advice and data on marine and freshwater environments — Centre for Environment, Fisheries and Aquaculture Science (Government fisheries body)Relied on for: Marine environmental science for UK shelf seas, including sediment character, coastal process and inshore monitoring.States this directlyCitation checked — the work exists and is described correctly
Related reading
- Depth as an environmental variableDepth is a condition, not a score. Light, gas volume, temperature and buoyancy all change together as you descend, and understanding that is what turns a dive into a plan.
- Identifying fish underwaterWater absorbs red light within a few metres, so colour — the character most people identify fish by — is unavailable at exactly the depths where the decision is made.
- Reading the tideCurrent, visibility and the safety of an entry are all downstream of tide state, which is why a dive plan starts with a tide table rather than a forecast.
- Turbidity and visibilityVisibility determines whether a dive is workable at all.
What these connections rest on (4)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Depth as an environmental variable
A physical claim about the diving environment, sourced to diving medicine rather than to practitioner report.
- DAN guidance on freediving safety, hypoxia and shallow-water blackout — Divers Alert Network (Water-safety or diver-training authority)Relied on for: Pressure effects on gas spaces, equalisation practice, barotrauma mechanisms and thermal effects in breath-hold diving.States this directlyPassage read and verified
- AIDA International freediving education and safety standards — AIDA International (Water-safety or diver-training authority)Relied on for: That structured training covers equalisation and depth-related physiology.States this directlyCitation checked — the work exists and is described correctly
Scientific evidence — 3 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Identifying fish underwater
A claim about the physics of light in water with a direct practical consequence, held to a scientific standard.
- The Fishes of the British Isles and North-West Europe — Macmillan, 1969 (Reference work)Relied on for: That morphological characters rather than colour are the reliable basis for species separation.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species-level morphological descriptions used for identification independent of colour.Background and mechanism onlyPassage read and verified
Reading the tide
A physical claim about what drives underwater conditions, held to a scientific standard on the tidal mechanism.
- NOAA Ocean Service tides and water levels educational tutorial — National Oceanic and Atmospheric Administration, United States (Government fisheries body)Relied on for: The tidal cycle and its variation, which drives the current and clarity changes a diver experiences.States this directlyPassage read and verified
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: That tide state governs the viability of coastal entries and exits.FishingHQ's interpretation of what it saysPassage read and verified
Turbidity and visibility
Visibility determines whether a dive is workable at all.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species and habitat data underlying this connection. Visibility determines whether a dive is workable at all.States this directlyPassage read and verified
How this compares
- A pre-dive safety checklistConditions are assessed before changing, including whether the exit will still be an exit in two hours.
- Bottom type, and what lives on itGround type decides the fishing and most of the hazards, so it belongs in the pre-dive assessment.
- Cold hands, feet and crampCold removes fine motor control before the diver notices, which is the ability to solve any other emergency.
- How visibility changes everythingVisibility sets shot range, identification confidence and whether a buddy can watch at all.
- Mask fogging, leaking and squeezeA mask problem in benign water is a nuisance; the same problem in cold or rough water is a reason to get out.
- Spearfishing wetsuits: open cell and closed cellCold degrades judgement before it produces obvious symptoms, which makes thickness a safety variable.
- Speargun length, and what it decidesIn three metres of visibility a long gun is a liability, because the shot is close and brief.
- Surface intervals and recovery breathingCold and current both raise oxygen consumption, so conditions change how long recovery needs to be.
5 more
- Surge, and how it behaves around structure Explainer
- Thermoclines, and where they put the fish Explainer
- Tide and current for divers Explainer
- When conditions turn during a session Explainer
- When to call a dive off Explainer
What these connections rest on (13)
Authoritative guidance — 11 connections
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
A pre-dive safety checklist
The conditions half of the check.
- BSAC diving safety guidance and annual incident report — British Sub-Aqua Club (Water-safety or diver-training authority)Relied on for: Diving safety guidance on pre-dive checks, buddy checks and equipment verification.States this directlyCitation checked — the work exists and is described correctly
Cold hands, feet and cramp
Cold is a capability problem before it is a comfort one.
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: Water safety guidance on cold water immersion, its progression and its effect on capability and judgement.States this directlyPassage read and verified
How visibility changes everything
The condition that changes the most things at once.
- BSAC diving safety guidance and annual incident report — British Sub-Aqua Club (Water-safety or diver-training authority)Relied on for: Diving safety guidance on visibility, buddy separation and conditions assessment.States this directlyCitation checked — the work exists and is described correctly
Mask fogging, leaking and squeeze
The margin changes even though the problem does not.
- AIDA International freediving education and safety standards — AIDA International (Water-safety or diver-training authority)Relied on for: Freediving equipment practice including mask fit, volume and equalisation.States this directlyCitation checked — the work exists and is described correctly
Spearfishing wetsuits: open cell and closed cell
Warmth is dive time and dive time is margin.
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: Water safety guidance on cold water and its effects on the body and on judgement.States this directlyPassage read and verified
Speargun length, and what it decides
Conditions decide the answer more than preference does.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Spear and powered spear gear specification and classification.States this directlyCitation checked — the work exists and is described correctly
Surface intervals and recovery breathing
The environment sets the interval as much as the dive does.
- AIDA International freediving education and safety standards — AIDA International (Water-safety or diver-training authority)Relied on for: Freediving practice on surface intervals, recovery breathing and cumulative oxygen debt across a session.States this directlyCitation checked — the work exists and is described correctly
Surge, and how it behaves around structure
The mechanism behind the hazard.
- Rock fishing safety and the declared lifejacket areas of New South Wales — NSW Department of Primary Industries and Surf Life Saving NSW (Water-safety or diver-training authority)Relied on for: Rock platform safety including swell sets, wave height variability and entry and exit hazards.States this directlyCitation checked — the work exists and is described correctly
Tide and current for divers
The rule that keeps divers out of trouble.
- NOAA Ocean Service tides and water levels educational tutorial — National Oceanic and Atmospheric Administration, United States (Government fisheries body)Relied on for: Tidal mechanics including lunar phase, spring and neap ranges, and tidal stream timing.States this directlyPassage read and verified
When conditions turn during a session
Why the decision is agreed on land.
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: Water safety guidance on changing conditions, cold water effects and decision-making.States this directlyPassage read and verified
When to call a dive off
The conditions record supplies what the decision is made about.
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: Water safety guidance on assessing conditions and on decision-making before entering the water.States this directlyPassage read and verified
Scientific evidence — 2 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Bottom type, and what lives on it
Habitat and safety are one subject here.
- Cefas science, advice and data on marine and freshwater environments — Centre for Environment, Fisheries and Aquaculture Science (Government fisheries body)Relied on for: Marine habitat science on substrate type, structural complexity and associated fish communities.States this directlyCitation checked — the work exists and is described correctly
Thermoclines, and where they put the fish
Why the interface is worth diving to.
- Cefas science, advice and data on marine and freshwater environments — Centre for Environment, Fisheries and Aquaculture Science (Government fisheries body)Relied on for: Water column stratification, thermocline formation and its effect on plankton and fish distribution.States this directlyCitation checked — the work exists and is described correctly
Safety reading
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Wading safely
Water-safety guidance underlying this connection.
- RNLI water safety guidance: cold water shock and Float to Live — Royal National Lifeboat Institution (Water-safety or diver-training authority)Relied on for: Guidance on assessing water conditions before entering, applicable to wading anglers and shore divers alike.States this directlyPassage read and verified