Got it from fort Benning down on a little river with a sandy bank. All of the pieces that I collected were baking in the sun on the sand. They were not close enough that they could have fallen from the trees and just fell and started drying. No, these had to have been washed onto shore by the river.
By the way, there was this MASSIVE tree stump on the sand. It was as tall as me and the root system was about as wide as I am tall. I'm 5'5" for reference. I'm going back next weekend to get some of the roots and maybe more driftwood.
As for it sinking, I've had pieces that wouldn't sink for weeks unless I weighed them down with rocks in which case they sunk within a few days. If anything I can fill my bathtub with water and put a couple rocks over them.