In Cozumel, I prefer shore snorkeling, so I'd split for the day while you scuba. Chankanaab is fabulous and so is Money Bar. I prefer Money Bar because it is a -cheap!- day. Easy cab ride from the cruise port, no entry fee, good prices on food and beverages, and good snorkeling. Chankanaab was really good snorkeling, but the entry fee makes it a lot more expensive.
At least for me, boat trips (which granted, I haven't done in Cozumel, since we always shore snorkeled, but we've done at lots of other islands on cruises) are never as good snorkeling because while the visibility is great, the reefs are too deep for snorkeling if you aren't a strong free diver. (We had boat loads of snorkelers over us at Paradise Reef, and I was wondering how much they were really seeing- since we were diving 25+ feet below them at the level of the reef.)
There are a number of dive ops that will do combined scuba/snorkeling trips though if that is what you want. Just email your desired ops and ask.