there's a lot of good shopping in Cozumel too. Don't be afraid to bargain aggressively. They will deal and if not there's another store right next door with the same stuff that will. The snorkling was the highlight of the the island.
From our resort on the mainland we could see Cozumel pretty well and we would watch the cruise ships go in and out all day and night long. The best was at night when they were all lit up