yea its floating cos its dry.
you need to give it a chance to suck up the water whilst its soaking before it will sink.
you can use rocks to hold it down then tho an that can help.
its always good to soak it tho as @ombomb says, all the wood contains tannins which turns your water a brownish colour (doesnt actually look that bad and isnt harmful).
but it in a bucket, barrel and change the water every couple fo days untill its clear or your happy its clear enough.
all wood takes a differeent amount of soaking too, i was lucky, i didnt get any tannins from mine but some can get alot.