I have never snorkeled, but I've been to Playa del Carmen which is near Cozumel, and I think the snorkeling is very good there. There's also an area where you can swim with dolphins.
I've seen someone who claimed to be stung by a jellyfish in Cancun. It seemed to be more frightening than dangerous in her case, and it was suggested that the jellyfish may have been attracted by her navel jewelry.
http://this-twisted-history.blogspot.com/