How I fog an I/O (Mercruiser/Volvo Penta) large displacement engines - only ever owned V8's
Bride at helm, boat on muffs.
Warm engine until thermostat opens
Run boat at idle, spark arrestor removed
Spray down each barrel with two cans, or two cans into the throttle body until engine starts to stumble
Ask bride to turn it off - keep spraying until engine dies
Turn master kill switch OFF
Open throttle to full
Ask bride to crank engine, spray both bottles for 5 seconds while cranking
Stop cranking, drain block and exhaust manifolds - be sure to poke out the drain holes
Put lugs back in
Pull large radiator hose and fill block with coolant
That is about it...not including the oil change. Although this is MUCH easier with a closed cooling system.
This method has always worked for me, and keeps me from pulling the plugs.