Good drift wood is usually the roots of trees and most likely from harder wood species. If that's not sinking straight away I hold it under water with bricks. I always soak my drift wood for a month or two before I use it in my aquarium. I have a garbage can I just fill with water and sink the wood in there. This forces the wood to get waterlogged and cleans or sucks out the brown tea coloring that makes the water brown. I change the water once a week mostly to keep the mosquitos under control. When the water eventually drains clear then the wood is ready for the aquarium. Most likely it wont be floating anymore.