The boiling will also help it sink....if it's been outside in the air that long it'll float like a champ. I just put a piece of lake driftwood in my tank, mine was too big to boil so I just poured boiling water over it many times and also soaked it in very hot tap water for a long time.
If it does leach tannins they won't hurt your fish, it'll just discolor the water, and some fresh activated carbon will help remove the color.