Update. Last night I pulled the water pump. No obvious cause for the leak - the o-ring was well seated, not pinched or cut. I cleaned up the block mating surface using a nylon kitchen thingie, followed by some metal polish. The mating surface is mostly bright, shiny metal, with a few discolored areas. I can't detect any roughness in the surface, nothing you can catch a fingernail on.
Put it all back together. Used a new o-ring, of course. Result: It leaks about 10 times worse than before.
If I had any hair left, I'd pull it out.
I'm gonna try this one more time. I've been lubing the o-ring with grease. This time I'm going to put it in dry. If that doesn't work, I'm either taking it to my mechanic or driving it into a lake, I haven't decided which.