Some tips from past experiences....
Dig your nail into the wood, if it's soft and your nail digs into it the wood is probably no good. You are looking for hard wood, possibly one that has been under water for some time. Many that are half exposed, half submerged have started to rot inside and you really don't want to take any of those home. Weathered wood, the really light stuff, can be used but needs to be tied down and won't become waterlogged in any reasonable timeframe. Tree roots are also very good, many sink on their own. As far as knowing what the piece of wood has absorbed during it's floating days really depends on where it was found, you obviously would not look for wood around a polluted waterway, nor one that has brackish water. I used to collect enough wood to fill a large trash can and leave it outside for months to soak, this usually gets rid of everything, then I'd boil it before use. I think there was a post some time ago about the actual types of wood that were good candidates, if I find it I'll post it.
We should organize a little collecting trip one of these days
Giancarlo Podio