Nice looking DW. That looks pretty aged and I would guess it's hardwood and you're ok. Soft woods would probably rot completely before getting that "weathered". Or else it's soft wood and that effect happened more rapidly. Does it feel soft at all or lose debris when you scratch it under water?
I've piled rocks on wood, and have mounted them to slate to keep them down. Most of the time mine has sunk on it's own after about 3 or 4 weeks.