I've been to Cozumel about 6 times. Lead a group trip on the last visit.
Without a doubt the best op I have found is
Blue XTSea. The only problem with Christi's op is that she books up real fast.
If she is booked up,
Blue Angel would be my second choice. The main thing to look for is an op with small fast boats. Unless you are staying way south, i.e. far from town, it's a fair trip to the dive sites. Slow cattle boats are not much fun.
If you go with either, please tell them I sent you.
James