Nina.......we had a great time snorkeling at Chankanaab; the water was very clear and we saw a lot of fish. This is a great destination; you could spend the whole day there.
If you only want to snorkel for a short time and then go somewhere else, people have enjoyed Dzul Ha. You don't need to book with a dive group/tour for shore snorkeling. It's easy to do on your own.
Snorkel equipment can be rented at both Chankanaab and Dzul Ha.