A PFD is a very good idea.
I just think you are deluding yourself if you think a snorkel vest would save you. Any water you are in that your snorkel vest will provide sufficient assistance, I can tread or float in. I can tread for at least an hour without getting tired (I've had to do that recently for water tests) and thus could tread for even longer than that. Maybe if I'm diving up north, that's an issue- but the sites I was near, there were so many boats it would be shocking to be in the water for even close to that long. But, if it was a several hour wait, I can float well enough that I can take a nap in a swimming pool while floating, in the ocean, I can't sleep because of waves, but can float indefinitely. (And don't go with the "but what if you are injured coming off the boat: if you were injured, you couldn't get the vest blown up either.)
If the water is very rough, a snorkel vest isn't going to do much. If the water is calm, it isn't necessary.
Have you ever worn a snorkel vest in the ocean, or are you just talking...?
Yeah, I can swim. No, I can't float that well and treading water for me is work, enough that I'd revert to Marine Corp Drown Proofing position - except that gains 10# of buoyancy by lowering the head under surface between breaths, not a good position when you want to be seen and not hit. So we're different, huh? I take a 3 mil suit in case I have to have it on a trip, but avoid using it.
So, maybe you should try using a snorkel vest and see how much difference it makes - except it may not be obvious if you're a human cork anyway. Mine is extra large and does provide a lot of buoyancy; I've tested it for that - ok?
I'd rather see boat crews offering PFDs as people board and helping them get them on if they want one, but we know they don't like to get them out. I really wonder how many boats actually have them in Coz, and if the DMs know where they are? I suspect this boat kept some in the forward area below, the last place I'd want to enter on a sinking boat.
Back to what we should be discussing, it seems all too common that boats tourist board on Coz as well as all over the Caribe...
(1) May be piloted by captains who don't know to watch out for water coming over the stern (ass end), and their mates & DMs may not either;
(2) Many of these boats are thin hulled and will actually sink if water gets inside, as compared to structurally buoyant boats that will just ride lower in the water if full;
(3) In some cases the captain may get off a radio call for help and other boats may be around, but in some - not either.
(4) And you never really know which way the currents and winds will push you if you do float?
And you're backup plan would be to float? Okee dokee. If a boat I'm on starts to founder, I may jump and suggest others may not want for the crew to think of that.