I've never seen one of these, but they sound like a great idea. I'm heading to the W. Caribbean on Dec 15th and would love to take one. Can I buy one in Miami or will I find one while on the cruise? I'll be stopping in Georgetown, Roatan, Belize City and Cozumel.
If you're looking for a snorkel vest, try looking at your local sports store. I'm sure if they have then they will be more reasonable than while on vacation. If not, I'm sure you can find them in any of the ports and even rent them while in port.
Good luck and I use them too and I feel much better with one on and can stay out there for a loong time, have fun.
Or try one of your local dive shops. Snorkel vests are not cheap, so I wouldn't invest in one unless you're going to be doing lots of snorkeling. They'll be available for rent in most Caribbean ports and on most snorkeling excursions sponsored by your ship for just a few dollars.