DS wore one two years ago (he was two). They never said anything. It just looked like a wetsuit. It was nice because it was December, so it kept him a little warmer at least on the walk to the pool.
Built-in floaty bathing suits are the best. My DD(5) has used her's many times at resort pools including SAB, WL and BWV. It has never been an issue. Now that she can "swim on her own" I am back to being wet and at arms reach! Have a great time!
My DD has a spedo swim vest (like a floatly suit, but you can take it off when you aren't in the water) She loves it and it really works for us. Much better then a life jacket because it doesn't go up around her face.
Anyway at a local pool the Lifegaurd gave me a hard time about it because it wasn't a aproved life jacket. The swim suits are like this, but it isn't like you can take it off and let the kid swim naked so they might have to allow it.
I don't think Disney would give you problems with it. Most likely I won't bring it because it takes a bit of room and we will get a loaner from Disney if possible.
The 1st time she would ever wear a life jacket is a vero beach and she loved it. (being able to swim herself.)