The dolphin pool IS on the chilly side, but it's not intolerable and when they tell you to do a full immersion and get it over with, they're absolutely right! It helps a lot, and you'll find yourself wanting to be neck-deep in the water most of the time. The day we went it wasn't overly warm, so it will probably be similar for you. Even so, it was a spectacular experience, and the park is so gorgeous it's a shame to miss it.
It seems to me they provided a choice of either a boyant lifevest or a wetsuit. If it's overly chilly go for the wetsuit, which will definitely keep you warmer.
We had about 1/2 hour exclusively with our dolphin (CeeJay was our dolphin, and he's just about as delightful as he can be! It was a real treat to have met him.) It doesn't seem like much time, and it does fly by fast, but it's such an intense experience that you will find yourself remembering it as being much longer.
Discovery Cove is really an all-inclusive experience, with the snorkeling reefs, the stingray reef, the aviary, etc, and don't forget that your meal is included! They have a wonderful selection, too, not just your standard burger/fries/chicken fingers, but some honest to goodness FOOD (and you'll need it! All that snorkeling makes you hungry!).
It's a bit on the pricy side, but if you can swing it, it's truly not to be missed.