Lots of good suggestions, thank you, but I don't think any of them are correct.
The end of the tube is closed and it is only a small diameter.
It is not a fishing rod/pole holder as there others on boat and the diameter is too small, with a closed end. It wouldn't be a good place for holding dock lines due to the closeness of the prop. There is already a grill on the other side of the boat and it is not for a table to cut bait as one of these is on the back rail already. There are also no antennas or outriggers in that area and it is not for shore power as the inlet socket is up near the front starboard side of the wheelhouse. it also isn't for a sunshade as the whole back deck is covered by a hard top.
I am still stumped, but appreciate the ideas!