There are restrictive laws for Ruerto Rico, I'm not sure about St Thomas, but in places like the Bahamas you can, with a fishing license, bring back 5 aquarium fish. There is also a law, called the Lacy Act, that prohibits animals from just being brought back across international lines so you have to have the fish inspected by customs when coming from some countries, I'm not sure if those places are included in that.
It is simple to find out, contact fish and wild life and also research the rules for fishing in the countries you want to collect. Most countries don't care about tropicals if you have a license and you don't take undersized game fish. There are no restrictions on most marine species being brought into the U.S. as long as you claim the fish to customs and you don't do it commercially. Way back when I used to be into rainbow fish and went 3 times to Australia and New Guinea to collect them and had no trouble bringing them back with customs declaration saying I had some tropical fish for personal tanks.
Also find out about the cruise lines you are using, they will probably have restrictions on what you can bring back on their ships and they may be much more restrictive then the laws. Just remember whatever you do do not try to smuggle the animals back into the country, that is a very serious offence and not worth getting any animal you may find.
Just my opinion, but with few exceptions (like royal gramma or black cap basslet) there are no caribbean fish that you can't get while on a FL vacation, and if you get a fishing license there are no restrictions on fish and motile inverts you can bring for home aquarium. You can't take coral, but you can't take it from any caribbean destinations, but you can collect tropicals here and take what you catch home with no fish and wildlife or customs needed as long as you follow the fishing laws.