Maybe they've changed it since your dad went. You float down the underground river in a life vest. We were disappointed because we thought there would be stuff to see in the river (like fish and stuff). That wasn't the case.
We thought about going in the snorkeling lagoon but it was very crowded and I'm not a fan of getting kicked in the head by a fin.
I have to agree that the maps weren't the best but at least the signs to some of the more major attractions were half decent (although we did walk in circles a couple of times).
We saw the turtles, manatees, jaguars and other animals. Don't expect the Aminal Kingdom.
Other than that we floated in the swimming lagoon for awhile (it was 90 degrees and I was glad to get in the water). Tubes to float in are free of charge.
Overall I would rate this excursion as "ok". If I were to go back to Cozumel I would likely go to either the ruins (although, I think I would have died from the heat and humidity if we had that day) or gone to one of the beaches recommended by people on these boards. It definitely would have been a lot less money.