Seasickness and boat days
Explainer · Last reviewed 2026-08-19
Ordinary, under-covered, and a genuine safety matter rather than only a misery — a seasick diver is dehydrated, exhausted and inattentive, which is three of the things that reduce the margin.
Safety: boat safety — can cause injury needing treatment.
Never dive alone, dive one up one down, and never extend a dive for a fish.
Seasickness is a mismatch between what the inner ear reports and what the eyes see. It is not a weakness, most people are susceptible in the right conditions, and it affects experienced divers.
WHY IT MATTERS BEYOND DISCOMFORT. A seasick person vomits and becomes dehydrated. They stop eating and drinking. They become exhausted. They stop paying attention to their buddy, to conditions and to their own state. Every one of those reduces the margin that breath-hold diving depends on, and the combination is worse than any of them alone.
It also produces a specific bad decision: a diver who feels awful on deck frequently gets in the water because the water feels better. Sometimes it does. But entering the water because you feel ill is a decision made by somebody who is already impaired.
WHAT HELPS. Being on deck in fresh air with a view of the horizon rather than below. Staying hydrated and eating something light before the motion starts rather than after. Medication taken well in advance rather than once symptoms have begun — and choosing one that does not cause drowsiness, because a sedated breath-hold diver is a different hazard.
WHAT DOES NOT HELP: looking at equipment, at a phone, or at the deck; heavy food; alcohol the night before; and getting in the water to escape the feeling without honestly assessing whether you are fit to dive.
When seasickness means not diving
The honest threshold: if you have been vomiting, you are dehydrated and depleted, and diving on that is diving with a reduced margin you cannot measure.
That is a hard call to make after paying for a charter and travelling to a mark — which is exactly the sunk-cost pressure the conditions material describes, arriving in a different form. The answer is the same: the pressures to get in are immediate and concrete, and the pressure against is a hypothetical you cannot see.
A reasonable rule agreed in advance with a buddy is that vomiting ends the diving for that person for the day, and that they stay on the boat as surface support rather than as a diver. That keeps them useful, keeps them safe, and removes the decision from the moment when they are least able to make it.
Medication taken for seasickness that causes drowsiness also ends the diving, for the same reason nobody dives on alcohol: the drug is affecting the judgement that the safety system depends on.
Safety
Never dive alone, dive one up one down, and never extend a dive for a fish. The full set of breath-hold essentials — training, the buddy system, conditions, equipment and visibility — is on the spearfishing safety page.
Boat safety — Can cause injury needing treatment
A seasick diver is dehydrated, exhausted and inattentive, and frequently enters the water to escape the sensation while impaired.
What reduces it: Fresh air and the horizon, hydration and light food before symptoms, non-drowsy medication taken well in advance, and an agreement that vomiting ends the diving for that person for the day.
Where to go next
Where it matters
How this compares
More explainers
Not related to this page — just the rest of the section. There is little else joined to this subject yet, and a short page should still lead somewhere.
Sources and evidence
- 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 fitness to dive, including illness, medication and boat operations.States this directlyCitation checked — the work exists and is described correctly
Why the 3 connections on this page are here, and the sources
Where it matters
Official guidance — 2 connections
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 page 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
The specific bad decision it produces.
- 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 fitness to dive, including illness, medication and boat operations.States this directlyCitation checked — the work exists and is described correctly
Boat diving
The setting where it occurs.
- 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 fitness to dive, including illness, medication and boat operations.States this directlyCitation checked — the work exists and is described correctly
How this compares
Official guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Breath-hold physiology for spearfishers
Why it is a safety matter rather than only a misery.
- 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 fitness to dive, including illness, medication and boat operations.States this directlyCitation checked — the work exists and is described correctly
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