We did the Atlantis Sub tour in Cozumel in 2004 with 3 of our kids who were then, 14, 7 and 5, the ALL loved it. DD5yr was excited about spotting "Nemo" and scaring a stingray that took off out of the sand on the bottom. DS(7yrs) was fascinated with all the fish down there. The rest of us enjoyed how peaceful and beautiful it seemed there under the water. You have a crystal clear view of everything without any lights on the outside of the sub. Learned that as you go down, you begin "losing" certain colors, I believe red was the first color to "disappear". It was a very interesting and educational experience for all of us. All 3 children wrote 'reports' on what they learned and saw on the bottom for thier teachers when we returned home. I say it is one of the better tours.
At the end, the guide tells you that only 3% of the worlds population has ever gone down in a submarine, and they give you a certificate saying that you were on the sub and the date etc. It was a nice souvenier!