This was the only place I saw jellyfish in the sea in Cuba, apparently giving off a fair sting. The lifeguard on the beach pulled these out of the water. Nice blue colour but , don't touch in the water.
**UPDATE - After Malcom's comment below and checking on Wiki and other sources, I can confirm these are Portuguese men o'war and as such can be extremely dangerous, and should not be touched even washed up on the beach, as the stingers contain venom that can persist for several days after the death of the organisms themselves.
These look like blown up blue condoms and will float on the water. If you see one on the beach (theres lots) give them a wide berth as the trailers (the long stringy things with the stingers, yes that is my professional description lol) will be just under the surface of the sand and will sting the bottom of your feet. And yes, the only relief from the pain is to get someone to urinate on the stings.
Is you see something floating on the water that looks like a nice blue balloon or bag, don't try to pick it up with your hands. These are jellyfish. A lot of tourist get caught by this and it hurts like crazy when it happens. It didn't happen to me but it did to my boyfriend. It might sound weard but the only thing to do when it happens is to get someone or yourself to pee on the area. This is what we were told by locals and later by a doctor.