Hello,
Went on a great snorkel excursion on St. Kitts, it was a full day excursion, with snorkeling at Smitten's Bay off St. Kitts and then a sail to a beach on Nevis.
This was on a catamaran, it was a ton of fun.
The life vests are compact so you don't feel encumbered but they are great, they enable you to take off your mask if you want and empty it out, while slowly treading water, without the feeling like you are going to sink. As was said before, do wear the vest, it is very helpful while you are in unfamiliar water. The currents can be strong even in a calm bay.
I hope you have a great time. Wish I could go back!!