I have used Don Foster's in GC. Typical cattle boat, but I enjoyed the trip.
<http://www.donfosters.com/> And also Eden Rock...Nice boat, and okay dive.
Here's a link to dive ops with reviews, in Coz: <http://www.travelnotes.cc/cozumel/links/diveshopreviews/general.html>
I have used Papa Hogs, Dive with Martin (both small boats/groups-great dives),
And various other operators.
My husband and I were on a Celebrity cruise last September that stopped in Cozumel for the day. We did not sign up right away for the cruise lines dive excursion and then got shut out of it since it was limited to only a few divers. The ship was charging $90.00 per person for a 2 tank dive which was a little on the high side. We both thought we could do better since there are so many dive operator in Cozumel. We got off the ship and there was a dive operator right at the pier called Dive with Martin and we signed up with them. Very convenient. It was either $60.00 or $65.00 per person for a 2 tank dive. We did not need to take a taxi. We left from a dock right near the ship. They took us to Palancar reef in a small motor boat. I think there were about 6 of us plus the divemaster. They were very good and I would dive with them again. The drift diving in Cozumel is wonderful! Have fun.