Mike, wouldn't the swivel shaft be out of the water when he is on plane? If that leaks it should do the opposite of what he is experiencing... would leak at the marina, not leak underway.
I am thinking maybe a pinhole leak in a water hose. When under load, the water pressure is going to be higher in the cooling system due to the pump spinning faster. Plus, you get some added pressure from the intake moving through the water at 30mph. It might be a leak small enough that it isn't leaking at idle.
Although, you did run with the mechanic and it didn't happen.
Risky, but I might take the engine hatch off completely and go run, with someone able to watch the whole engine compartment (from above of course)