The ones you'll encounter during a snorkel expedition are very small, around 2" to 3" across. Their sting is like a mosquito bite and not the crippling agony of a Man-o-War or some of the large jellyfish. When you get back on the boat, you'll be doused with a vinegar solution that will neutralize the sting and kill any jellyfish that might be stuck in your hair.
To be honest, they're more of an annoyance than a real problem.
We were at CN last week and yes there were some jellyfish. We saw one swimming in the water, there was one that was dead in the water that a couple of folks pulled out of the water. Also, we were told that one night on the cat cruise there were a lot of jellyfish in the water, most of them were dead, but a few people got stung. You just have to be careful.
We were at CSA Aug 17-22. We did both the day and night snorkling, and did not encounter any jellyfish. However, while playing in the water around 6pm, about 20-30 feet from the water line, both my girlfriend and I were stung. This was the first time either of us had the experience, so it was a little startling. Thankfully the security gaurds on the beach gave us help, and sent us to one of the bars where we asked for a large glass of Rum. We rubbed the Rum all over our skin and after about 30 minutes we were fine. We didn't hear of anyone else getting stung.
We were at CN Aug 22-29 and yes there were jellyfish in the water. I got stung twice on my arm and I never saw the jellyfish. We did see alive ones on the cat cruise and like Blondie I saw someone pull a dead one out of the water. The ones we saw in the water at CN were as big as dinner plates.
oh, and I forgot to add an old natural remedy that really works for stings....rub that special "golden liquid" that we all produce from within.......I think you get my drift. It works wonders!
Hah! I was stung on a night dive last year at CSS, and was begging someone to pee on my legs... One of the divemasters was like, "No mon, there will be no peeing on de boat!!" He quickly provided me with some alcohol, which didn't really help much... but I think he was trying to keep someone from dropping trou and relieving themselves on my legs...
Like Amy said above.. the light was on the dive boat, and it attracted them. That was the only time I saw jellyfish the whole week.
We were there July 19 to 24 (6th trip to CN) and yes there were large jelly fish - so we initiated taking turns on the "JF" watch. They are as jersey noted - dinner plate sized so you can see them. CN was still beautiful - cannot control Mother Nature.