There are many tours and activities in Cozumel, most include the beach or ocean. Swimming, snorkeling, SCUBA, jeep explorations of beaches and everything in between. We opted for a ride in a submarine, the Atlantis. We were able to dive to 108 ft along the reef system right along a shelf that drops off to about 2800 feet. The ride did not feel close, rough, or scary. The ships crew kept everyone lose with cheerful banter and facts of the reef. Making our approach, you can see the sub and the tender that takes care of the subs needs. The sub is electric and has enough juice to run for 24 hours. Rated by the manufacturer to dive as deep as 800 feet, the sub is allowed to carry civilian passengers to 150 feet, but normally just 100 feet. I think this for 2 reasons. First is that the reef around the island is only about 100 ft deep give or take a few. Second if you meander out past the shelf, you encounter that 2800 ft drop. I do not think they wish to take passengers out over the shelf as this would be quite the thrilling (albeit short) ride before you could come to a full stop. Of course you never feel the bump of hitting bottom. At least over the inside shelf, the sub would be in easy range for a rescue - should something happen - of course. Seriously though, we never felt any anxiety over course of the dive.