Catch a taxi to Chankanaab National Park. I like this place because it has something for everyone. It does cost a little more because of that. Without swimming with the dolphins, it is $31/ person. 4 of our group went scuba diving with a guide after a 5 minute how to lesson. ($60) They said it was fantastic. You can also snorkel from the shore. Lots of chairs and shade. There are dolphins, manatees, a sea lion show, and a botanical garden. The only thing it doesn't have are the inflatable water slides that some beaches have. It is very clean and largely wheel chair assessable.
I also like Paradise Beach but the snorkeling isn't good there. It has beautiful grounds, great view of the water, chairs are decently spaced, lots of shade, great waiters, a pool, and inflatable water toys.