depends on the kind of wood......i have malaysian driftwood that sunk right away but i also have another kind (dunno what it it) that HAS TO be anchored.............
The key is to tie down floating wood with weight and fishing string and hide it in your gravel or sand.... and then every now and then check to see if it is fully water logged
It depends of what kinds of wood you use. I think they all have to sink sometime. They float because the wood has a series of air cavities inside. Once this cavities are full of water, the wood sinks.