Run a nylon rope through the hollow and tie the loose end back onto the rope.
Leave enough length on the tag end to tie off on a bank anchor of some sort.
Push the log into the water close to where you want it to end up.
If it sinks, cut the loop in the rope and pull it out.
If it floats, tie it off to a bank anchor.
It will, eventually, absorb enough water to lose bouyancy and sink.
You can then remove the rope.