Geo-fish is a good shop, also Playa Scuba is is a great operation. The diving is either the animal oriented dives off Playa del Carmen or the plant life off of Cozumel. My best dives were with Aldora Divers on Cozumel. You can take the early ferry over to go out early or they will work with you to coordinate with the cruise ships to go later. They utilize steel 120 tanks so you will have lots of gas. The day I went the seas were a little rough so we went in their larger boat, not their six-pack. The DM’s and boat crew were awesome. I will use them again.
The other diving option is the ceynotes. They are the inland caves located between Playa and Tulum. I dived them almost exclusively two years ago since the harbor was closed nearly every day due to large waves. Each ceynotes is a little different, yet stunning.
I hope you have a great time.
Dennis