Cozumel IS very popular with out transatlantic cousins, especiallt Texans, as it's only a couple of hours' flight. It's known colloqially as Houston South
The americans are either excellent divers or numpties who only dive when on holiday every third or fourth year. You'll meet both kinds at Cozumel. The diving is pretty good by Caribbean standards (nowhere near as good as the best parts of the Red Sea). The u/w topography is very impressive and there are some strong currents. It's not unusual for divers to be lost and never found. I was in a group of 8 with a guide who were adrift for 45 minutes before another dive boat (not ours) spotted us and picked us up! Do take that smb and a good whistle, and make sure you have enough thermal protection – you can get very cold after 45 minutes drifting after a dive, even in comparatively warm water.