almost all Keys dive ops allow snorkelers, but not necessarily on all trips...e.g.-if they are doing a deep wreck....many ops do a "dive-only" trip in the morning and in the afternoons mix it up with divers, snorkelers, DSD, SNUBA, etc.....usually you can get paired up with another diver on the boat, but sometimes a DM may be required for an extra couple of bucks if no one else is available...my best advice is to call around to the various ops and ask them these same questions since they all operate a little differently.....have a great trip!!!!!
p.s.-when you have your plans set, make a post on here and someone may be willing to buddy up with you, myself included