And the answer is ...
It WAS kitty litter making the silt. Came back from another sailing trip on someone else's boat, and there was water in the tank again. Marked the water level, pumped it out into the cockpit.
The next day, the water was at the same level ... things that make you go hmmmmmm!
So I went to the store and got food dye, and poured some in the scupper, and ... the water in the tank turned red.
So I called my friend who had helped me with a dirty fuel tank. He had to disconnect hoses. At my boat doing something else, I went to run an errand, and he climbed back into the hold ... and found that after he reconnected everything, he had reconnected the water tank to the scupper hose instead of the water fill hose. I've been gone so much this summer, and I knew something was up with the tank and I just didn't use it this summer. You'd be surprised how much you can work around an unusable water tank if you have to!
So now it's all connected the right way.
My big concern now is that I *know* kitty litter clay has gone through the water pressure pump. I'm thinking, clean the tank out thoroughly, put fresh water in, suck some up into the pump and ... leave it there, give the clay a chance to re-dissolve? There was a pot in the sink when I drained the tank through the galley sink, so the water flowed in an arc. It actually made a hard "dam" of clay in the sink when it dried. That can't possibly be good for the water pump. Any thoughts on this? Should I put anything in the water to make it more likely that the clay would dissolve? If so, what could do that, that would not damage the pump?