Re: WTF is this?
based on the fact that the fuse is a "slow-blow" that is typically used on loads that have high in-rush current on starting, such as a motor, and the heavy gauge of the wire I would say it is meant to operate a motor.
Has it got power to it?
I wouldn't be surprised if you had a macerator pump hiding somewhere. If not, that looks like a great switch to control one in future.
1979 Santana 30 Tall Rig