Okay, I am not familiar with the Volvo Motor
that the OP mentions, but exactly how does the water flood the engine past the Raw Water Pump? On my boat the water enters the boat through a thru-hull past the shutoff seacock (when open) through a short hose up about 8 inches to the Raw Water strainer, out of the Raw Water Strainer through a hose the drops about 7 inches and goes into my engine mounted Raw Water pump.
My Raw water pump, as many others here, has an impeller that has 6 rubber fins. These fins block the water from flushing through the engine unless they are turning and they don't turn unless the motor is on. So unless the OP's motor and Raw pump are set up differently, I don't see how surfing down a wave or another way of achieving a speed that "scoops" water in, will force water past the stationary impeller.
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