Shouldn't be a problem! Disney seems to be much more flexible and kid-centered than other hotels in this regard. My kiddos wore their Speedo swim vests each and every time on the Poly Volcano pool slide - probably 100 times, LOL. Many other places won't allow you on a slide unless you can touch at the bottom or pass a swim test, no vests allowed, etc.
I also noticed this at Blizzard Beach. Other than the big speed slides, the kids can do everything!! Mine were 2 and 4 (almost 3 and 5) and some of the slides they went on were huge! Family at home just couldn't believe it from the pics. Again, around here, they're not tall enough for most. This is a big thing for me, b/c my son is quite short for his age, and as of this summer is swimming very well. At the pool we go to, you have to be 48" to go on the slide. Some of his friends went this year, probably all will be able to next year, and he probably won't be able to until he's 7 or so. Disney was nice for him - much fewer restrictions due to his height - rides too.
Oops, sorry for my tangent!! You should be fine with the float - have fun!