Not the holy grail, or a pot of gold, or anything like that, but my fresh water leak! It's great because now I can fix it, but annoying because it's a connection I had already checked. Another positive is that it's in a spot that's pretty easy to get to.
One of the threaded parts was the cause of the leak, so I went down to West Marine to pick up a replacement. They didn't have the same part, but they had something called a quick connect fitting that does the same thing, only simpler. Great!--except that my old French boat has things in metric sizes, and West Marine didn't... Oh well.
In the end, the best solution looks to be to seal the leaking threads with a fast-curing permanent adhesive. Problem is, it takes 24 hours to cure, so no running water until tomorrow night!
Update: It worked great! No more leak.