The wood you can buy has in fact been in water, and has since dried out. Very rarely does it take 6 months to sink. Generally it takes a few weeks.
If you go and find the wood yourself, make sure there is no bark on the wood when you put it in your aquarium. You can also bake it in the oven, and some people will boil it. I think just soaking it in water in a bucket or other container works fine. Just soak and change the water daily for a week or so has worked for me. Do not use fresh wood. Make sure you find it on the ground. The drier it is the better. If it still has sap in it, this will come out in the water.