There are several TSs on Cozumel --- they all trade in RCI.
In addition to La Ceiba, there is the Reef Club (which is now Wyndham - IIRC), Coral Princess (really a hotel) and Parc Royal.
I have stayed at both the La Ceiba and Parc Royal. If you are going to Coz for diving, I recommend the La Ceiba over Parc Royal: access to the dock is much easier.
Reef Club (where I have not stayed) should be an ideal location for diving. It is much closer to the major reefs and has its own docks.
elaine