I was stung by a jellyfish at Chankanaab Park on the 8/30/03 Western Cruise. I was snorkeling fairly early in the AM and had seen one rather small, clear jellyfish several minutes before and was leisurly kicking along the surface when a tenticle wrapped around my right thigh and gave me a series of stings, about as painful as mild bee stings.
I have a history of bee sting allergy and travel alone so I swam, kicking with only the other leg, toward the life guard tower. I was wearing a lifejacket that I had brought along with my own snorkle gear so I wasn't really in a panic.
The lifeguard assured me, in broken English, that jellyfish stings are not usually serious unless you are allergic. He applied vinegar from a spray bottle and I remained close to the guard tower, with my epinephrine syringe close at hand, for about a half hour then returned to enjoy my day at the park.
By evening time I had nothing left from the incident but a couple small itchy spots on my leg and an interesting story to tell to my tablemates at dinner.
Chankanaab Park was very beautiful and relaxing. I loved looking for iguanas. The snorkeling is lovely for anyone of an "advanced beginner" stage or beyond. You can't touch the bottom much. I always bring a ski vest style lifejacket. I helps greatly when adjusting your mask and snorkel in deep water and if you ever get caught in a strong current (or ambushed by the marine life) you at least remain floating on the surface.