OK here is, I hope, the end of the story. The mechanic checked the fuel pressure, and according to him it was way too high. He saw it spike to 80 psi, and it's supposed to be a steady 43 psi.
The only thing strange about that is my son had checked fuel pressure also and it was about 45 psi for him, so it looked normal.
So the solution was to replace the pressure regulator ... parts and labor were about $200. I can certainly live with that, given what it could have been.
The mechanic noticed the oil was about a quart too high. I noticed that too but didn't connect that to anything. He said it also smelled like fuel, and his theory is that the high fuel pressure caused gas to leak past the rings into the oil. He highly recommended changing the oil, which I agreed to. Just thought I'd mention this in case anyone else runs into this problem in the future.
Anyway his diagnosis is certainly consistent with way too high fuel consumption, black plugs, and white smoke. He even said the oil slick I saw behind the boat could have really been gas mixed with other crud from the engine being blown out the exhaust. I asked him if the computer threw any fault codes, and he said there were none. I'm pretty sure there is no fuel pressure sensor (he thought so too).
So I just brought the boat back today. Don't know when I'll be able to take it out for a test, but I did run the engine in the driveway. No white smoke whatsoever and no surging at idle - all good signs!