Cancun? go snorkel on the west side of the island, towards the southern tip where you can cross over, its calm, plenty of fish and stuff there, watch out for the jet skis.
Have a great time, I know two places in Cancun where you can snorkel well without exposure to waives. One next to Hotel Camino Real and another one next to Club Med. Major snorkeling is at Rivera Maya where reefs are more abundant.
Just to let you know sponge is a very simple but delicate creature. Once you expose sponge to air it's good as dead. The air gets trapped in sponge pores and eventually sponge dies from starvation. Just keep this in mine if you plan to bring some home.