I can fully relate here. I was a dive instructor and yacht crewman down that way for a bunch of years. I've hauled more bikes down south than i can recall, and a few of them back up here !! ( I'm sure there are several of my old bikes still being pedaled, and even raced, around Cozumel, Playa Del Carmen, etc.!!) The above advice to watch out of local drivers was spot on (don't get me started !!)
I've had bikes with me on a couple boats, but unfortunately, a sail boat doesn't have a lot of extra room (I've known "blow-boaters" to like those rinky little folding bikes, for light sightseeing and grocery runs in port).
My best advice is to tell him to suck it up, and to keep in mind that for the rest of his life, while he's out bicycling, he'll be reminiscing about his time sailing the Caribbean !!!