Oh ghod, my aching brain... A bullhead fish larva jumped two inches and entered the thru-hull for my head sink and died, blocking it nicely. Maybe they have a thing for beer-urine?
Anyhow, I disconnected the keel-engine bond and interposed my DVM last night with the positive lead on the motor and the negative lead on the keel. Previously it (the bond) had been connected (to the motor) for a couple of days, and recall that the zinc was installed on Saturday as I was doing the anti-fouling paint -- one coat after scraping off the freeloaders. So the keel is painted, with the exception of the bit of iron behind the zinc. I did not paint the zinc.
With that arrangement, I measured +100mV. As the evening progressed, the DVM reading slowly climbed to +160MV. I should note that (a) my temporary LED strip cabin light draws 240mV, and I tend not to use the remaining incandescent fixtures. The fresh-water pump is used sparingly. (FW system needs a small expansion tank. TODO.) So I knock off for the night and shut off the DVM. Today I was busy starting the top-down back-to-front Cleaning Job so did not ogle the DVM until a few minutes ago. So I turn-on the DVM and it show -90mV, which has slowly climbed/fallen to -95mV in the last half-hour. Clearly there is something slightly fishy going on.
Clearly my prior hypothesises is/are somewhat in error and more investigation is needed. Time flies like a banana, et al.