well Jane, I'm big and my guide still held my hand!!
I have been to several of the snorkel spots. While Hol Chan and Shark Ray Alley are not to be missed, I enjoyed Mexico Rocks so much more. It's very quite, we were the only boat that day, it's beautiful, current is not a problem--it can be very strong at Hol Chan-- and you are always close to a place where you can stand up (just be very careful where you stand!) We saw sting rays, sharks, cudas, a beautiful eel, and tons of very colorful fish there. I think Mexico Rocks is a must-see. We went with Oliver from Seaduced, and he is THE best guide if you ask me.
Just get in the water... it's all good.