Why you get seasick
Seasickness is a mismatch between what your eyes see (a stable boat interior) and what your inner ear feels (the boat moving in 3 dimensions). Your brain interprets the conflict as poisoning and triggers nausea. The cure is to either reduce the motion (impossible at sea) or reduce the conflict (look at the horizon, stay on deck, sleep horizontal).
Medication: what actually works
The most effective options ranked by liveaboard divers worldwide: 1. Scopolamine patches (Transderm Scop), apply behind the ear 4 hours before boarding, lasts 72 hours. Prescription required in most countries. Side effects: dry mouth, blurred vision. 2. Stugeron (cinnarizine), sold OTC in most of Europe and Asia, taken every 8 hours. Mild drowsiness. Very popular with European divers. 3. Dramamine / Bonine (dimenhydrinate / meclizine), OTC in the US, taken before and during the trip. Drowsy. 4. Ginger capsules or ginger tea, natural option, mild but real effect. 5. Sea-Bands (acupressure wristbands), works for some, placebo for others.
Behaviour tips that work as well as drugs
Stay on deck and look at the horizon, never below deck reading or scrolling on your phone during a crossing. Eat plain carbs (bread, crackers, rice) before and during the crossing, never travel on an empty stomach, and never eat heavy or greasy food. Stay well hydrated but avoid alcohol the night before a crossing. Sleep flat in your cabin during the worst of the swell, lying horizontal removes most of the inner-ear conflict. Sit in the middle of the boat (least motion), not at the bow or stern.
When to take which type of vessel
If you're prone to seasickness, choose: a steel-hull boat (smoother in chop than fibreglass), a larger boat (40m+ rides better than a 25m boat), and itineraries with shorter open-ocean crossings. Avoid: catamarans on heavy chop (can be jerky), sailing yachts that heel a lot, and remote routes like Cocos (36-hour crossing) or Socorro (24-hour crossing) until you know how your body reacts.
If you do get sick
Get out on deck immediately. Look at the horizon. Breathe slowly through your nose. Don't fight throwing up, once you do, you usually feel much better within an hour. Ask the crew for a Stugeron tablet or a ginger tea. Skip the dive if you're unwell, dehydrated and exhausted divers are a DCS risk. Lie flat in your cabin between dives if needed.
