As the more experienced fellas on here have been sayin'...you have a water infiltration issue. The reason it didn't rear it's ugly head on any other outing was because the amount increased over time, after each use...your last timeout, it over-flowed with the water/oil mixture and created the oily foam and milky oil you had. Somewhere, you're getting water into your crankcase. If I was a betting man, I'd bet you have a crack in the heads, block, and/or intake manifold. If your "mechanic" did a pressure test, it was most likely a compression test as the crankcase pressure test is a little more involved.
You'd have to verify both winterization processes as being done completely correct, otherwise I'd be thinking cracks from freezing? Is this boat from Dallas?