Make them now. The tours are small (like 8 people) and fill up quickly. And it is absolutely worth it. It was the highlight of our last trip. We spent a while learning about dolphins and the research performed with them at the Living Seas (even got to watch an experiment where the dolphin responded to various visual stimuli by honking a horn with his snout in different rhythms....fascinating!) before wading into the water up to the waist and a little higher to get up close and personal with two of the very friendly, adorable dolphins. We were taught hand signals to do with them that provoked Sea World-like tricks. It's even worth it just to see the top of the Living Seas aquarium...that place is HUGE!
And the majority of the money goes toward conservation efforts and not Disney's pockets. I can't recommend it enough.
I did the Dolphins In Depth back in the Spring of 2000 and enjoyed it immensely! Just make sure you know what the full experience entails. I think it's changed the format a little bit since I did the tour, but our group spent a lot of time in the Living Seas pavilion with a dolphin trainer or other Living Seas CMs BEFORE getting in the water with the dolphins. It was like a guided tour of the pavilion and you get to learn a lot about the research that they're doing there. We all enjoyed that portion a lot, but (of course), the dolphin encounter was the highlight.
You wade into a shallow portion of the tank and the dolphins come up to you. We had two dolphins (Bob and Toby--no longer at Epcot) between our group of four. We got to give them back-rubs and bellyrubs while the trainers talked to us about their biology and why encounters like this help get the dolphins accustomed to getting examined by veternarians and other trainers. While cameras were not allowed on the tour, they shot video of us in the tank and gave us a copy as part of the package. You do not *swim* with the dolphins, but we all thought just meeting them up close was totally worth the cost of the tour. I highly recommend it!!!