Dolphin lagoon is 72 degrees, the grand reef is 77 degrees, and the fresh water pools are around 85 degrees.
Gina is correct in that everyone's definition of cold is different. But you will find the lagoon and reef to be on the chilly side, especially if you don't have much body fat. I'm a guy and I've done days with just swim trunks and the Discovery Cove vest and been fine. I've also done days with the wetsuit. The wetsuit can get uncomfortable and hot out of the water and you may find yourself a little on the cold side with just the swim vest.
My personal preference is to just wear swim wear and the DC vest, as I can always take a dip in the pool or lazy river to warm up. On another note, you can opt for just the vest and try things out and always go back for the wetsuit later if you want. The DC staff are excellent about catering to you!