During our Mexican vacation we spent a day at Xel-Ha. An amazing eco-aquarium. Everything is natural, the fish are not in tanks (nor are they afraid of you ). SOOO cool.
We swam in a cave.
I dove (jumped, actually) off a cliff a few times.
We tubed down a mangrove path and swam down a "lazy" river.
The water was amazing.
Xel-Ha is an all inclusive park. $105 per person (through our tour company at the hotel) covered transportation to and from, rental of all snorkel equipment (bring $20 deposit for snorkel equipment, you'll get it back), towels, lockers and all you can eat and drink (including beer and cocktails). The only added expense (besides the shopping I did on my way out) was purchasing their bio-degradable sunscreen. Being an ecological preserve, standard sunscreens are, understandably, not allowed.
For extra fees you can do things like swim with dolphins, manatees or go on an underwater "sea-trek". While they all seem fun, these activities will take up part of your day. We were perfectly happy to simply explore the park on our own on our first visit. Even with this approach there were a few things we didn't get to do. I guess we'll just have to visit again.