There are snorkeling tours that go to Isla Mujeres and Cozumel, both islands near Cancun. You can go to both of these places by yourself for much less money, and if you are a sightseer rather than a serious snorkeler, or if you are already going on several other tours, you might just want to go on your own. 

To go to Isla Mujeres costs $13.50 US for the round-trip, and you can find coupons for a 10% discount. The water-taxi leaves from the marina near the "Fat Tuesday" bar in the hotel zone. 

To go to Cozumel, you must leave early from the bus terminal downtown for Playa del Carmen on the coast. Once there, you take a ferry to Cozumel. The reason for leaving early is that the bus ride takes a couple of hours, the ferry stops running relatively early in the evening, and the ferry can be busy so you may wait a while to go over. 

There are a variety of tours through the jungle (really a swamp with jungle vegetation) surrounding the Cancun lagoon. You can either pilot your own jet-ski, or cruise on a boat. You sail through the twisty passages in the swamp until you reach a spot where you can snorkel the rest of the afternoon. 

Experienced snorkelers will probably prefer the jungle tour where you ride the jet-ski. Beginning snorkelers or those travelling with their families might prefer a boat tour like the one AquaWorld offers to its private island, "Isla Paraiso". After going through the jungle by boat, you spend the rest of the day on a man-made island with change rooms, a restaurant, and free food, drinks, and snorkeling equipment. You can snorkel by yourself or go on a guided tour of the reef. 

You can also snorkel off the beach in the southern part of the hotel zone. Get off the bus at the Westin Regina hotel and head towards the Club Med. 

There is also great snorkeling at Xel-Ha, which is often combined with a tour of the nearby
Tulum ruins. 

Warning: Don't apply any suntan lotion before snorkeling as you are not allowed to swim with it on it's toxic to the coral reef.