The mechanic is working with me, he totally acknowledges now (after some initial resistance) that there's obviously a problem unrelated to "break-in”....I should note here that the rebuild did uncover a couple major problems that needed attention--e.g., new, oversized pistons were required--but who knows how long the engine might have continued performing well enough until a rebuild was absolutely needed? It’s like going to a surgeon for hernia repair and the surgeon ends up fixing your bad knee (that you were doing just fine with).
The boat was not covered up for the winter..plus some of the fuel in my tank is probably 2-3 yrs old, though I do treat it regularly (Stanadyne). I'd asked my mechanic several times--before the rebuild--to check and replace the fuel, but he always dismissed this as a primary cause for the white smoke, said he had checked the fuel & it was OK. But I don’t know exactly how he checked the fuel, I know there are different methods.
I’ll have him re-check the injector pump timing--good suggestion.
Thanks everyone.
Matt