Beaches are nice, water is warm. Beware of timeshare pitches. They are everywhere including the airport. Every discounted tours you see is part of a time share pitch.
There's not much to see snorkeling in the Cancun resort area. Further south in Playa del Carmen have cheap boat rides for snorkeling. I saw some boxfishes just swimming near the surface in Cancun while having dinner outside. If you stay longer make a day trip or two day trip to Cozumel which is one of the best snorkeling sites around.
You can't bring and fish or coral back since you have to go through U.S. Customs on the way back. Within the US like Hawaii you are able to bring fish back which is what I did.