Sure enough, after much spot testing with a hose and prodding around, I found it.
That tiny little hole is all it took to have water dripping all over the place. It's an old termite hole, and it had bored all the way through 2" of teak to the underside, but had been plugged with mud and crap until we scrubbed it hard. A syringe and a little epoxy fixed it.
The joys of timber boats. (I complain, but love em).
