During my first trip to Cozumel, I used Sand Dollar
I've been to Cozumel quite a few times. Here is my list of DO's
Dive Paradise (twice).On time, safe. I would recommend for beginners. The DM were very attentive and tried to make sure everyone had a good time.
The DO at Iberostar: Way too cautious. They treated everyone like it was their first pool dive. Midway through the week they did lighten up. I didn't feel the DM's had the same experience level with the area as the other operations.
Aqua safari: Very nice operation out of the Plaza Las Glorias. Only two days with these folks but I felt comfortable and safe.
Pepe's out of Coral Princess: Wonderful! Fast boats. The DM's went to great lengths to lead us to sights and critters. My wife and I both enjoyed our time with either of the DM's they sent with us. It is a small operation with personal service.
Sand Dollar: Four days of diving each worst than the last. Won't do that again. Inattentive DM's. Back to same area twice. DM's Buddy low on air, DM 60 ft away. Overall a very poor experience.