rocks are in my opinion the best way. you can eather wedg them into the wood or i have drilled slat before and yoused ss screws to screw it to the wood plus if you screw the wood to a rock you can position it in different ways on the rock to what you like.
Like stated above you can soak the wood to get it to sink, but that can take a while. You can place a rock on or over the wood to hold it down until it absorbs enough water to where it will not float. Or you can tie a rock to the wood.
I asked this question a while back. Someone recomended that after soaking if for a few weeks, I should take it all out and let it dry out complete. I kept it out two weeks, and then put it back in, and a couple of days later it all sank. I don't understand exactly how that worked, but it did, so you could give it a try.