It's an interesting experience and fun to do (once) for a new diver because they get to see alot of things in the context of a single dive early in their dive career. It is very well done as you would expect from Disney (it is a nice shirt) but is relativly expensive for a single dive. I believe it cost about $140 per person. They shoot video and invite your family in to view it (and purchase) after the dive.
The lenght of the dive is a function of how all the critters behave - usually 30-45 minutes. In addition to the dive you get an interesting behind the scenes tour of the tank operation. The tank itself is approximately 200' across and 33' deep. It's really a big doughnut with the muggles looking out from the center observation area.
Interestingly (at least when I did it several years ago), the sharks are actually free swiming in the tank with you, and it is the dolphins that are behind bars in a portion of the tank. At that point in time, Disney and the US Navy were involved in a joint dolphin breeding program......one to the Navy, one to Disney, and so on.
The name of the Epcot operation/attraction is Dive Quest, and if you have interest, make sure you book well in advance because the number of divers per day is pretty limited.
Cheers,
Sieg