Check to make sure the Square Edge of the O-ring groove didn't Cut some of the O-ring off when they installed it. You can use a little RTV Silicone to insure it seals. But if it leaks a little it won't Kill the engine. You will learn How to deal with a Dunked Boat before long,,, It WILL Happen. You did the right thing to a point by using ARO and\or WD-40 for the initial flush but ALWAYS start the engine after flushing and allow it
to get to operating temp to insure All Moisture is evacuated from the Crankcase.