Hi, bogwood needs to be well soaked in a bucket (for weeks) to remove tannins (which would stain your water). During this time the bogwood will become 'water-logged' so will naturally sink.
Drift wood, depending on what it is, may need weighting down for quite some time.
I often end up using a stainless steel screw to attach mine to a piece of slate. I drill a hole through the slate with a carbide bit and screw into the wood from below the slate. The screw is hidden completely. The size of the slate depends on how much weight I need to sink the wood. A sprinkling of substrate on top of the slate will hide it if you don't like the look. If you have trouble finding slate, like I did, you can get real slate that is made up to be floor tiles. A hammer and a little time will make the slate tile whatever size and shape you want.