Sounds like an older early small scale sea chest manifold valve and if in bad shape replace it with modern equivalent bronze valves or have it rebuilt. To tell more I would need sorry to say better pictures you could email them if need be
It was designed so that when the seacock is closed there is a smooth surface on the bottom of the boat. Yes, it looks like a basketball pump, but it is definitely a seacock. I'm guessing it was a C&C custom piece. A couple O rings (missing in the photo of the handle) provided the seal.
I've emailed South Shore Yachts to see if they know anything.