You must go snorkeling at Xel-Ha during your stay at the Mayan Palace. Take the resort golf cart to the road and catch the Mayab bus which will take you to Playa del Carmen where you can catch another going to Xel-Ha and Tulum. Or you can take colectivos there. You will still have to get off the road near Playa and catch another colectivo to Xel-Ha. Xel-Ha is a bit costly -- 20-40 dollars depending on whether you want the all-inclusive (food, gear etc.)
You really won't want to take the time to eat. Don't combine this trip with Tulum. You'll want as much time here as you can get. Bring your own snorkeling gear to cut down on the cost. Wearing of sunscreen is prohibited, as it kills the coral and fish. You can swim with dolphins here for an extra 80 dollars or so. Snorkeling is everywhere, but the staff will tell you the best spots. The really big fish are by the swinging bridge. You will also see sting-rays here.
You can float down the river in an innertube, which we found to be exhausting because the current is often against you. There is a beach, a small cenote where you can swing from a rope, and a lagoon. There is a train that can transport you from one side to the other.
We had the most fun just snorkeling for hours. It is like a huge aquarium. Get there before noon because then the people really start coming.