The crew administers oxygen and first aid immediately, contacts a dive-medicine hotline by satellite, and arranges evacuation by boat, helicopter or coastguard to the nearest chamber.
Every liveaboard runs an emergency action plan and briefs it on day one. In practice the sequence is: get the casualty out of the water and onto the deck, start 100 percent oxygen immediately, keep them warm and horizontal, take vitals and a dive-profile history, and contact a dive-medicine hotline such as DAN via satellite phone.
The hotline physician decides whether the case needs recompression
If it does, the captain begins evacuation towards the nearest port with chamber access, sometimes meeting a fast tender, coastguard vessel or helicopter partway. On remote itineraries this can take many hours, which is why oxygen supply on board is sized generously.
Cost is the part divers underestimate
Helicopter evacuation, chamber treatment and repatriation regularly total $20,000 to $100,000. Standard travel insurance rarely covers diving-related evacuation and almost never covers hyperbaric treatment. Dedicated dive insurance does, and it costs a fraction of a single trip.
You can reduce the risk substantially: dive nitrox conservatively, add safety stops, keep ascent rates slow, stay hydrated, avoid alcohol between diving days, and be honest at the briefing about how you actually feel. Crews would much rather you sit out a dive than treat you afterwards.
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