The aquatic experiences you've heard about take place in the giant aquarium in the seas pavilion at Epcot. There are three guest tours in the aquarium: Divequest, Dolphins in Depth, and Aqua Seas.
Divequest allows SCUBA-certified guests to dive underwater in the tank to interact with the marine life. Divequesters can also see out of the tank into the Coral Reef restaurant. It's a thrill for the diners to wave at the divers in the tank!
Dolphins in Depth lets guests have detailed discussions with dolphin trainers and scientists to learn more about these incredible animals. There is an opportunity for guests to interact with the dolphins in the water, but this is not a "swim with the dolphins" experience that you might find at other theme parks.
Epcot Aqua Seas is a snorkeling experience at the top of the tank. SCUBA certification is NOT required.
There are age requirements for all of these activities. For complete information on this, as well as for pricing and to make reservations call 407-WDW-TOUR.
If you're going to the Typhoon Lagoon water park, you might also be interested in the
Shark Reef attraction. This is included with your TL admission ticket and there is no need for reservations. If you're looking for a starter snorkeling experience, this is a wonderful option.