Unless the mechanic is sufficiently "confident" that he'd be willing to reimburse the cost of a short haul if it turns out the Max-Prop is fine, I'd seek a second opinion...That was my original request to test the engine first. Perhaps do the compression test and test the injectors. I agree that tt is far cheaper than haul out the boat. But the mechanic was so confident it was MaxProp problems.
He's getting paid for his time anyway, I'd have serious reservations about enlisting the services of any mechanic who wouldn't want to take a quick trip out into open water first, and observe first hand the symptoms you're described... Hell, even running the boat in gear at the dock, under load, should be able to closely replicate the problem, no? Has this guy even HEARD the engine run yet?
Someone's gotta ask, sooner or later... Doesn't the actual CAPTAIN on this $1+ million boat have any clue how to proceed with this sort of deal?