The tour is $140 and yes, it is worth it. It is very informative and the time spent with Calvin & Rainier is well worth the price. The money from the tour is donated to the WDW Conservation Fund.
I just did the tour again last week. They now take a picture of you with your arm around one of the dolphins (instead of a video) and you still get a t-shirt (sizes are S to 4 XL).
The water is not too cold -- it's about 70 - 72 degrees, but it is "refreshing" when you first step into it. However, after a few minutes, you are used to it. The odd part is seeing some of the other fish who can swim through the fence coming to the platform and swimming around your feet. You have to be careful where you step!