Many tourists will go to X-Caret (www.xcaret.com) for snorkelling, the underground river being one of the most interesting places I did snorkeling. They also have a little beach and other places to swim and snorkel a little.
The zoo in interesting, and a spectacular show is a must see at the end of the day with dance, traditions, costumes, and the story of Mexico. But if's always full of people and a day is not enough to make all the park.
But Xel-Ha is even better place for snorkelling. They are centered on that : a little hike but most interesting are the River descent on and snorkelling in the giant closed bay so you can swim with incredibly numerous fishes all size. It's safe (they give you a vest) and well organized (lockers, etc.)
We took the best : we did both :)
By the way, you can economize by taking the little white van that takes people Mexican, like tourists along the main road. They pass many times per hour and will stop if you're waiting near the road.
The Tulum area is known for the cenotes as a place to snorkel, but there's also snorkeling off-shore. It's too far away to swim to (for most people) but for about $20 a person you can get on a small boat with a few other people for a snorkeling excursion.
The trip is about a few hours, and they'll take you past the Tulum ruins - which look really nice from the water - then a guide will take you around a pretty cool reef with lots of coral, fish etc...
The snorkeling isn't as good as say, St. John in the Virgin Islands, but it's still cool.
There are several caves in the area that make for great snorkeling. The small local ones are cheap and great and you can rent gear there. The big fancy ones - like at Xel Ha are expensive and commercial, but also pretty impressive.
We went snorkeling at Xel-Ha, an ecological park in the Tulum area. It consists of a freshwater river that runs into a large estuary. There are mangrove along the river, which you can tube down (you MUST wear a life jacket, but if you don't want to, it's okay according to the guy working there lol). Once you're in the lagoon, which is huge, you can turn in your tube and snorkel around in it. It really is massive, and you will not get to explore the entire area. There are fantastic fish, I saw a a 4' 'cuda, got a pic of it, I'll have to find it. Also saw rays, all kinds of beautiful tropicals in bright colors. It isn't a reef, but we saw plenty of fish, and the calmer waters are great for kids or beginners. The fresh and salt water mixing cause cloudy conditions occassionally, but it generally wasn't a problem. Along the river there is a cliff to jump off of, and some random (difficult) physical activities to try. Great fun.
Make sure you schedule an entire day for this place, as there are tons of things to try, we didn't plan and didn't see half of what they offer. There is also a beach, turtles, a butterfly farm, snubaing, swimming with the dolphins. Make sure you take biodegradeable sun screen because they won't allow regular. Go to a health food store to buy it. Also, make sure you slather it on before you tube down the river. You are the sun's captive audience for about 2 hours, and you will regret it if you don't. My husband had blisters the next day.
A ceynote is a freshwater pool with fish and plants. The peninsula that Tulum is located on is made of limestone so there is no surface water; the rain percolates through the limestone to underground rivers. Every once in a while, one of them caves in, a sinkhole forms and you have a Ceynote. The Grand Ceynote is a fairly large one and has lots of cool fish, caverns, and vegetation to explore. It is not very expensive to get in and a great family trip and/or nature excursion.
Bring biodegradable sunscreen and have your suit on under your clothes as the changing room is nowhere that you would want to spend any time...
You are not supposed to bring food or drinks in. Just to let you know, there is no scenic spot once you exit because beyond the road on either side is private property secured by large fences. You could try eating close to the entrance, away from the ceynote. There are trees to eat under and they may not be as rigorous about the no food rule.