I was trying to avoid having to run the engines. With the closed cooling system, I am only concerned about flushing the heat exchanger and exhaust. Superflush makes a system that does this for about 3-4 grand for twins. I was looking at plumbing a quick connect fitting between the seawater pump and heat exchanger. My concern is fresh water reverting through the heads if the motor is off.

not sure how much water would pass thru sea water pump...mine worked great...but never tried without running the engine....but I did not have fresh water cooling...