I'm not sure how long you're going to be in port. Most of the really good snorkeling takes a while to get to. One site you might check out is http://www.sunnydayskeywest.com
They do some great trips to the Tortugas, but I don't know if you'd have time to do them. They might have something closer as well. John Pennycamp Park is great for diving, but that too is a good ways away from Key West. Hope this helps.