We freaked out today to find water running across the floor outside the head.  My first thought was that one of the though-hulls in the head was leaking,  but turning both off made no difference.  My next thought, though I don't know why, was to put the slurping tube of my oil change pump down the shower drain and start pumping.  I did that and got some horrible putrid stuff out of the shower sump.  At that point the water stopped running across the floor, so I felt better. 

Next question:  Why had that just started to happen?  Where did that water come from?  Well, I had just been pouring hot water into our hole-in-the-counter fridge to melt out some ice from the bottom.  Could that be it?  Another gallon into the hole started water flowing across the floor again, so yes.
Would I have to hand pump the sump every week or so from here out, I wondered?  There's a switch in the head marked "shower sump", but it never seemed to do anything.   April, maybe try hitting the breaker marked "spare?"  Sure enough, it works!  So no I don't have to hand pump the sump, but yes it would be a good idea to run the thing periodically.  

Another boat-ownership lesson learned the hard way. Or at least the scary, gross, and ultimately hilarious way.