We got a taxi to chankanaab park ($10 each way). Rented snorkel equipt. (can't remember how much, but very reasonable). snorkeled, enjoyed the magnificent beach, nature trails with mayan historical village and relics, and reasonably priced food. The souvenirs we found in Chankanaab were much cheaper than the ones we found later in town.
It was a wonderful morning. I wish we would have skipped going to town to buy souvenirs as it was stressful, crowded, dirty, and unnecessary. And this is coming from a new yorker!!
Doing this on our own was completely EASY, non-stressful, and enjoyable. It also saved us $$ because the excursion there was more expensive and it was limited in time. We were able to spend whatever time we wanted there. Taxis were a set price and were always there. I would definitely consider this.