sorry i'm late, hope these'll do...Nice find, can you post a couple extra pictures close up?
I also think it’s a Maul but very water wornJust my opinion, but It's the shape especially the back (poll) end, the flat sides and the bit shape that makes me think it is a maul, it doesn't look natural to me. Groove may be natural, not pecked. If I was a native and found it I would immediately think it would make a good maul.