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.