You can rent single and double tubes at SAB< but you better get there pretty early in the morning. I think they cost about $3 an hour for the singles, but they don't charge you more if you don't return them, so they disappear very shortly after pool opening and aren't likely to be returned. They only have about 10 tubes or so for the whole pool, and there's no limit on how many each person can rent, so good luck.
They also rent noodles, but those go fast, as well.
You can buy smaller tubes, which aren't large enough to float on. I don't know if there's a rule against bringing your own, I don't remember seeing anything in the pool that wasn't bought there.
I don't remember seeing any kind of floats at other Disney pools- they may be against the rules, not sure. Even at SAB, I've never seen any lounge-type floats. The pools are not large enough to make such things feasible.