I was born in the Caribbean and live in the US now but have been all over the islands and never been stung, while living down there I found the most Jellyfish activity in the colder months but have been known to sing in the summer months, most avid beach goers keep a small bottle of vinegar just in case. I now wear a shorty wet suit in the winter month and have less skin exposed to the cold water and Jellyfish .
Like flacousa said, most jellyfish activity is in the colder months for us here in the Caribbean. I've noticed that Sept through November are really bad months where you will see jellies in the hundreds-thousands here in the keys. I tend to stay out of the water during these months until March/April comes around, but if it's pretty hot wherever you go I would get in anyways. Have to enjoy the vacation.
http://en.wikipedia.org/wiki/Aurelia_aurita
This is the jellyfish you will most likely encounter. The stings aren't really bad but you might have a couple temporary bumps/marks here and there. And it will be really itchy. Might last a couple days. This is odd but if you don't have vinegar just pee on the infected area, it will relieve it in most cases, but I would chose this as a last resort. Just hope the urine is acidic lol and don't put freshwater on it...
As you notice they are almost transparent so you might bump into a few without noticing.
http://en.wikipedia.org/wiki/Portuguese_man_o'_war
Now this is a Portuguese man o war, stay far from these, the sting is a lot worse, not fatal but you will be in pain. You won't find them in large numbers, not as common, and they float so it's easy to spot.
But I don't recommend getting close, their tentacles can be 15ft-20ft long, If I remember correctly... I've seen someone got stung by one of these and the pain was so bad and we didn't have vinegar or anything. So he told someone to pee in a cup so he can pour it on himself... I've never been stung but have seen other's multiple times, doesn't look like fun....