FYI, I just got back from a 3 day cruise in the Carribean. The cruise line did not tell us that the swarms of Thimble Jellyfish in the water can sting and leave you with a burning, itching rash. They were everywhere, and people were picking them up, etc. The kayak guide on the private island told us later that day that apparently they can give you the rash. He said that the rash appears later, and is activated by freshwater. I have seen information on the internet that supports his comments. If you see the thimble jellyfish in the water, you may want to think twice about getting in!
There were little jellyfish all over in Cozumel when we stopped there on our cruise in December. They were about the size of a silver dollar. It only seemed to bother those with sensitive skin and the rash, irritation was gone in about an hour or so. I know that I brushed up against a few of them in the water and didn't have a problem but my cousin who has very sensitive skin had a couple of little ichy spots for about an hour.
Major rule to live by is to never pick up anything that looks like clear plastic in the water unless you can see that it is definately a plastic bag. Unless you see the writing on the bag assume it is a jellyfish and it will ruin your vacation.
We had an issue with them in Nassau and Great Stirrup Cay. I don't have sensitive skin at all, but I still have the rash. I got in the water at GSC on 4/13. The gentleman we sat with at breakfast the next day mentioned his daughter had a rash, also.