All wood will (or should) sink once it is waterlogged, and different wood takes less or more time to do this. Soaking the wood in a bucket of water sometimes works; the larger the wood the longer it will take. Attaching the wood to a piece of slate and then burying the slate under the substrate is another method that can work, though the larger the chunk of wood the larger must be the slate and sometimes this can cause other problems. Wedging the wood down with rock can also work, this is better for smaller pieces than larger.
This is one reason I like the Malaysian Driftwood. This may have other names, ironwood, etc. It is very dark brown and sinks immediately as it is heavier and denser.
Byron.