Originally Posted by DrB
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.
Contrary to popular misbeliefs and misconceptions raw water pumps are NOT positive shut off valves and water can and does leak by them.
Just ask the guy at my boat yard who last fall shut his engine off with the raw water intake hose in a 5 gal bucket in the cockpit. Instead of pulling the feeder hose out of the bucket before shutting down the engine he left it in the bucket. Once the engine stopped it began to siphon because the bucket was higher than the siphon break. It siphoned this water, filled the water lift muffler and then when he went to crank his engine it was seized or water locked with water in the cylinders. All this happened THROUGH the raw water pump..
In a perfect situation a raw water pump can stop water flow but not all pumps are perfect, not all impellers are perfect and not all pump bodies or face plates are free from wear..
Never count on a raw water pump as a valve..