Big truck tubes have no butt scrapes and you just let the tube take you were it wants to go. The largest ones used for quarry work are so big that your hands will not reach the water. You "swim" by hanging on and kicking in the water or off the bottom in still water. Not recommended.
Small truck tubes let you sit in them and do the back stroke. Requires good shoulders. The usual size used are around 10.00R20's. Consult with your local tire dealer.
Large car tubes are useful if you have a life preserver. They let you stay submerge in the water all the time during a hot day.
Plywood shaped like a disk and tied in the middle gives you a non sinking bottom with a butt protection plate. People have found that rope bottoms stretch too much and hang up. I use this, but, it has failed to chatch on. Those who do not like the butt plate say they can't vary their positions on the tube. (Through the tube, under the tube and hanging on the side.)
Life Jackets (not the neck ones) provide safety, resting pad and depending on the type some wanted or unwanted warmth. The smaller tube and a life jacket works for me.
Most people use truck inner tubes. They hold up well and are cheap if you can find them. Beware of buying a tube which has rotted. Look at the edges where it has been folded. Pull or bend those areas. You not patch rotten tubes. Tubes left in the sun are also no good. Sun light ones have a faded look. Some people are adept at getting used tubes for free or pay to get them patched. Expect to patch used tubes, that is why they have been scrapped. I understand some have paid up to $10.00. Beware of the air filling tube. Some point to the middle, others point to the side and they all have various lengths to the tubing. Either style is ok as long as it does not scrape you when you are sitting in the tube. It is not necessary to have a cap on the filling tube.
Small rafts the type you lay on with your feet hanging out and floating beds also work but, do not allow sitting.
Test for leaks before the trip. Can you do the back stroke in it. Blow them up and leave them in your garage or crawspace for awhile to see if they leak. During the cool night the tubes will deflate some. If you have a extra one, I will take a few in the canoe for spares. Failures on the river is normal. I do not bring a pump.
PS: For a proper fitting tube picture see the Wall Street Journal May 22, 2004 Page 3D. OR at lest that is the day I pick up the paper at the library.