To ensure the driftwood have a better sinkable effect , you’d better pretreatment the driftwood before you put the driftwood into water:
1)Boiling in water for 1 to 2 hours with soda or salt ,the water color will become dark cause it releases tannins
2)Boiling the driftwood with water again ( recommended 3 times, 1 hours each time ) until the tannins totally releases