Go to Chankanaab and pay $10 per person and stay for most of the day. Good Snorkeling, swimming, clean, watch the dolphins, drink, eat, etc..
Cab to Chank was $10 per carload.
If it get's real hot, take cab into town and go to senor frogs or Carlos and Charlies for late lunch and then do some shopping. Whole day isn't too expensive.
You can see great pictures of Chank at www.maps-of-mexico.com