I have used float tubes and pontoon floats to fish rivers and the one thing I have noticed is you need to makes sure you can move around and have room to cast. On some floats, when trying to make longer casts, your line hits the water because you just don't sit high enough in the river. Obviously you could try other casts, like a roll cast, but I have never had very good luck with roll casts when you are trying for a longer cast. My advice would be to find a friend that has one and try it out before you purchase. That's how I eliminated a few options.

That depends on how much whitewater you plan to float... and I assume you are talking about a one-man model. You don't want to float more than class 2 in these anyway. The high end model for bumpy water would be Outcast's Ferrari (yeah, they really call it that)... an extremely stable and durable boat. One step down from that is the Discovery series (also Outcast), which allows you to stand and cast. For the best/lowest price check out the Creek Company's ODC models, which are about half as much ($300-$700 rather than $1000-$1300). I have one, and it's fine for lakes and fairly flat rivers. But if you want a boat, for a boat-type river, you should pay the extra for the extra stability and durability. I've been happy with Outcast. My two cents. Hope it helps.