[GCAT9110]

Obviously you have a engine coolant heat exchanger/radiator. If you lost 2 gallons +/- you should start with a cold engine, remove the radi cap, add a gallon and let the level come up but remain well below the radiator cap rim. Start the engine and let it warm up, it will eventually burp and the air will bleed out then the coolant will seek its own level, add as you need and top off while the engine is hot. Install the cap and run the boat under load, any excess will dump to the the reservoir. Pull the cap and check the level again once the engine has cold soaked.
Frank