This fitting is on the port side of a 16 foot Chris Kraft Cavalier

Does anyone know what it is or what it does?

Do you have a wider shot. This looks like an ameba! BTW, could be the base of some sort of mirror? Compas? Or maybe an ameba?

Looks like an ameba to me!

" noun,plural a·me·bas, a·me·bae [uh-mee-bee].

  1. any of numerous freshwater, marine, or parasitic one-celled protozoa of the order Amoebida, characterized by a granular nucleus surrounded by a jellylike mass of cytoplasm that forms temporary extensions, or pseudopodia, by which the organism moves, engulfs food particles, and forms food vacuoles."

I do like the Woodyboater spelling of amoeba :smile:.
Anyway, I have seen these through hull fittings before. I think the shape is to deflect water away from the hole when underway to prevent siphoning.
My best guess.