All wood sinks in time but time might be years in some cases. The drier the wood, the better (though it takes more time to sink intially, it's safer). You'll want to avoid certain wood types with a lot of sap or bark on them. Neither is good for the aquarium, especially the sap though. Also, make sure there are no critters living in it or that it has fungus or other growths. It would be good to scrub it down with a toothbrush or something similarly bristled and to be on the safe side, bake it in the oven and/or boil it before giving it a good soaking.