I read a great article on still hunting a few months ago. One of the methods discussed was using waterways to move silently through the woods, and with out leaving as much of your own sent behind.
I don't know if I'd like sitting on a float tube all day. Movement is too restricted, and simple tasks such as taking a whiz would be a pain. I'm assuming you mean a product like those waders with a big tube that lets you float while fishing.
A good alternative might be a stationary float with a ground blind on top. Find where the deer come to cross or drink and place a guide line from one shore to the other. Feed that line through some PVC attached to the railing of the float so you can pull yourself out to the middle and back. The only thing I'd worry about at that point is falling asleep to the lulling sounds and motions of the water.
