The best snorkeling tour in Cancun in boat i know is here with us. Of course on board a luxury vessel departing from Cancun. The best sports are candenas reefs, el meco, MUSA, farito reef, La Amada sunken ship, manchones and Garrafon.
The best tour for snorkeling depends also on the weather. If its sunny it would be much better. Currents are also a big factor. Manchones is hard when current is strong. In that case any other reef would be fine.
Best reef to snorkel in Cancun i think it should be Manchones, then MUSA. Full of fauna and coral reef. But just if current is week. you can see even sharks there, turtles and bif fish.
you can choose any yacht from this list of luxury yachts