I would say upper rudder of some boat since the draft markings start from top to bottom (it looks like a stern light fixture on the top too).
Have to be fairly big to have draft markings that deep.
If you click on the link and then click on the picture you get a bigger picture. The part I think you say is a stern light fixture appears to be a hinge pin.
Also if you compare it to the trash can behind it, the rudder if you will doesn't appear to be very big.
I don't know about the lumber but it does look like a fiber glass skin.
And those draft marks definitely add to the mystery.