This tour company operated from the Grand Mayan resort. They have an office on the beach. I was able to book a tour with them easily as the sales staff came by the pool with information and sign up forms.

I signed up for snorkel boat tour(65 USD) to Maroma Reef with 6 people but 5 of them cancelled an hour before the start time. Lee Corp Aquatics set me up with a snorkel tour guide on a Jet-ski, took 15-20 minutes to get there. They also provided life jackets and water/soda. The excursion was for about 2-3 hrs. The snorkel guide knew exactly where to look for and find great sea life. On the way back I got to "drive" the jet-ski, it was awesome.

I would recommend this to all guests at the resort.