Do a google for Richard Sowa, or Spiral Island... This guy took this concept to an extreme. (just noticed that matt has a thread about him, coming number one of the similar threads under this one)
I spent some time building boats, and can tell you this plastic bottle thing is super easy, but also really temporary. The elements will wear out a plastic boat like this in a few months. Even using big 200 litre blue barrels, you should calculate a 6 month life to be safe.
Also a really good thing to know (if you dont already), is Archimedes' principle, short version is that buoyancy is about displacement, and in constructing your boat, you can know that 1 litre of water (its cool, cause its the base measure for all volume) is equal to 1 kilo of buoyancy. A two litre bottle 2 kilos, and so on. So if you make a boat from 500 2 litre bottles, you can effectively float one metric ton (at the surface level of the water. You could safely float, say, 800 kilos, on a calm lake.)