Cancun is beautiful place. But i didnt see any reef stores. But tons of pubs with disco light even during daylight.
Take the catamaran ride, they'll take you snorkling. You'll see lots of fishes and some (not much) corals. The water is pretty clear. But becareful, they'll liqoured you up on your way to it. So, by the time you swim, you'll be pretty buzz.
Also, becareful on signing up for discounted or free tour in the Airport. Or you'll be stuck for a few hours on a timeshare presentation.
I'm sure you'll have fun..