Brad,
I can't promise you. I really can't. I'm just /totally/ guessing...
20000 MFD is a pretty big capacitor in terms of its capacitance. But, with a 50V DC working voltage (WVDC), it wasn't used on anything 120VAC, e.g. an air conditioner. It is in the engine room attached to an engine. And, 50VDC is a nice rating for something that will actually be used at 12VDC.
The upshot is that I don't think it was there to help kick-start a motor.
With a 20000 MFD capacitance, it would be right there in the range of what one would use to filter DC coming off of an alternator back in the day when really good DC power was needed for computers and electronics.
If I had a guess, and this is a total guess, that looks to me like a late 1970s to early 1980s, maybe mid 1980s capacitor. I'm thinking the boat once had Loran-C or some other sensitive navigation electronics and that capacitor was put in as part of the installation.
Then, sometime thereafter, the electronics were long gone and the alternator needed rebuilding, and they never hooked it back up. Maybe the Loran died, or was moved to a new boat, or sold for fuel money. Or, or, or.
I guess what I'm saying is that the cap is about right for filtering an alternator for sensitive electronics, I think. And not right for much else I can think of that would be right there on the engine. And, some early 1980s era stuff was right there in the sensitive electronics category.
So, if it were me, I'd take it out. It isn't doing anything now. And, it isn't doing anything that needs done today. And, for all we know, someone disconnected it for a reason.
But, this is just me guessing at it.