The quickest way to disinfect I'd to boil it but that won't make it sink the best way that I know to make it waterlogged is time it just has to soak for long periods of time sometimes months but there is always the option of tying it down to some type of rock/slate and burying the rock/slate in the substrate to hold it down for the time being but I would say just let it soak to get the tannins out of the wood so that it won't discolor the water although sometimes it can take a really long time to release. I know there are people on here with a little more experience though so I am sure they will chime in.