So I began to wonder, what are you really supposed to do if your dive boat starts to sink?
In my experience, one advantage to diving in the US is that the boat operator is required to give a Coast Guard mandated safety briefing before the boat leaves the dock. Most of my diving in Key Largo has been with Rainbow Reef and every time I've been on one of their boats, we get the full briefing: where the life jackets are located, how the life raft works, what the EPIRB does, where the O2 kit and fire extinguishers are, how to use the radio to call for help... and a request that if we see anything wrong, tell the crew and give them the chance to handle it.
I've dived regularly with Waterhorse Charters in San Diego... and we get exactly the same briefing (well, the key points are the same... the jokes are different.) Most of the boats I've dived with out of Long Beach do the same (but there are some that have been a bit lax.)
My point: I use that information as a standard for a safety briefing when I leave the US. It's also been my experience that most places in the Caribbean don't mention
anything related to a safety briefing. So I take it upon myself: where are the life jackets, fire extinguisher, O2 kit, radio? Make a note of these, if they exist. And if something goes wrong, follow the instructions given on that safety briefing you had on your last US dive boat.
Worth also mentioning: obviously a wetsuit or a BCD can keep you buoyant, but it's very misleading to suggest that a BCD is a life preserver. A CG approved life preserver is designed to keep the person floating with their face out of the water and be immune to failure (which is why they're solid foam and not inflated.) A BCD, while nice, doesn't provide the same level of safety. (With that said...yeah, I'd grab my BCD also if I was jumping in the water.)