These guys have been getting great reviews for the past couple of years and
the reviews just keep getting better.

http://www.bluextseadiving.com/

Laura,
I just returned from two weeks in Cozumel doing just what you have planned.
I checked out a number of shops and decided on Diving Adventures
http://www.divingadventures.net/english/default.asp for the following
reasons:
1) They weren't doing cattle boats
2) I was impressed with their personal and personable service
3) They were competitively priced
4) They did their open water dives away from the town beach, which isn't
attractive at all
5) They were very accommodating to my schedule
Their lead instructor, Mac, a brash, high-energy Brit, was a bit
overwhelming, but that may just be because I am, too.  He knew his stuff,
was full of entertaining and educational stories, and made me feel very
comfortable in the water.  I was pleased enough with DA to continue with
them for my "Advanced" cert. and ten other dives.
Also, if you're looking for an inexpensive, yet clean and handy, hotel in
Cozumel, the Posada Partita on the west side of 15th av. between Calle
Rosada Salada (sp?) and Calle 1 Sur charges 300p/night for two queens, bath
and nice common area where you can meet other divers and travelers sitting
around in the evenings.
I'm sure you are going to enjoy your time in Cozumel.
Padeen