I'd put it in a tub filled with water first and see what it does. Sitting underwater in a lake or river is one thing, but ime sitting outside doesn't necessarily mean it won't release heavy tannins once submerged in a tank.
A little sanding and you may even be able to identify what it is (by grain or smell). I had a piece of bogwood once that was old looking when I got it and then it sat around outside for a few years before I decided to make something with it (I carve bowls, etc.). The moment I started to grind on it there was a strong cedar smell, it turned into a nice cedar bowl. I had no idea what it was to that point, just an old gray piece of wood with an interesting shape.