First thing to do is see if you can press your fingernail into it. If you can, it will eventually rot.
You can boil it to get out all the tannins.
Then sun dry it if you want to be extra cautious of things living in it. Boiling should take care of it though.
Please be aware depending on the size of the wood and type it may take a long time to log over and sink on its own. Some wood will sink in days, some bigger pieces take years. You just have to be patient.
Keep soaking it underwater till it sinks or you can screw a metal plate onto the bottom of it after a while if it really doesn't want to sink.
I piled rocks on top of mine till it eventually sank on its own.
Hope this helps.