Something used for leverage to pull up...
A round ball on a rope, think crab trap bouy, and you clip the other end under something, maybe to lift the dagger board? Or to raise a bucket out of the cabin? Or used to tilt the outboard? (I'm thinking that might be it...)
Or, vice-versa, maybe used to pull down the furler/forestay to pin it by holding the drum in the round part and hooking the clip ends under something to push down on the handle end and force it down?
EDIT: Could have been used to help pull-start the outboard...
EDIT EDIT: it's weathered on one side, so that side may have been exposed to the sun. (Or to bilge water)
I would agree with the lever idea of pulling the furler drum down for pinning. The double notches would fit on each side of the single forward cleat. Then one could lay a ss rod or heavy screwdriver over the notches and under the cleat. This would give the leverage one would need to pin the forestay alone. Distance between the notches and the round cutout looks about right for the job. Pretty smart if that is its purpose. I like it.
I also think its a furler pinning lever (err.. does this boat have a furler??)
I can see the two fork ends being inserted over the top of the furler drum from the front of the boat. The "round indent" would fit over the fuler drum and then maybe a loop in a rope (or???) farther back on the deck loops around those two notches on the fork end.
Then from the front of the boat, you could pull down on the single stick end and pull the fuler into place to be pinned. If the back loop (or however its attached) is placed right, it would put the furler tang right in place for pinning.
I just did something similar (posted earlier) and I think as long as the wood doesnt bust, that could work nicely. Assuming that this is for pinning a furler..