Ya gotta love it when somebody posts a vague question about something he is completely ignorant about, and then chides those trying to help but are asking for more information.
There is nothing vague about my question given the vagueness of the object itself...even the yard where they hauled her doesn't know what it is for. (and this is a major yard, south of Charleston, that has seen hundreds of boats my size before)
And I wasn't "chiding" anyone....if I knew how to post a photo here I would...but I guarantee you it wouldn't do a bit of good to do so, as all you would see is exactly what I describe.....an enclosed rectangular box with black bottom paint on it stuck to the underside of the boat ! No obvious means of attachment, no exterior holes or vents...just a black box.
I am guessing that it must be held by a thru hull fitting on the inside of the hull and then was caulked around the edges and then bottom painted, but the boat is an hour drive away now and I can't just hop over there to look further on a whim....so posted this on the hope that someone would say "Oh yeah, I had one of those years ago as well....it's the ___________ for the _________"
FWIW, it is so close to the keel that I have not seen the "keel side" yet....perhaps on that side there is a screen or intake of some sort.....but no way to take a photo of that area...maybe next time I'm at the facility I can put my hand in there and
feel something on that side.
Wish I could remember the brand of the watermaker as a look at it's manual might give a clue...I just remember the watermaker is insanely complicated...and yet I would be completely mystified if I find no intake on the keel side of the mystery box as that would almost certainly rule out watermaker possibilities.
And yet if engine related you would think there would be two of them, one on each side (I have two generators as well, original equipment)