We have taken the Atlantis Sub Adventure in Cozumel and Grand Cayman (same tour company). It is a submarine carrying about 20 passengers plus crew. They transport you by boat to where the sub is waiting. Transfer from a bobbing boat to a bobbing sub. Take a narrow ladder down to the inside of the sub (NARROW as in better be on the slim side) watch your head, low ceiling. Sit bumper-to-bumper in the sub. If you don't mind cramped/confined spaces, this was a great adventure. We were down at various depth for about 20-25 minutes. Saw lots of wrecks, fish, etc. We had a great time and will do their operation wherever available.