@MISkier I really doubt it...think about how far the water would have to travel, plus the exhaust risers are way above the waterline. Also, the intake and exhaust valves regulate the valve timing in regards to piston position.
Even if the exhaust valves are open there’s no vacuum because the valves are open to the exhaust system and atmosphere. An engine is a air pump. Everything goes one way (unless it runs backwards like a two stroke can).
If it was my engine, I would do a compression test, differential cylinder pressure test (leak down test), look REALLY hard at the raw water pump (condition) and exhaust manifolds (pressure test them) for cracks, then move onto cylinder heads (cracks or blown head gaskets, compression and diff cyl pressure test should discover those issues) and finally block for cracks. If there’s that much water getting in it’s a pretty big leak. I would look REALLY hard for signs of severe overheating at some point. Trueing up the manifolds does nothing.
If there’s water still getting into all 8 cylinders, it’s a severe leak and should actually be easy to track down.
Water in one or two (next door to each other) would lean towards a blown head gasket or cracked head.
If it stopped leaking, compression is decent, has decent oil pressure I would run it for 5-10 hours and dump the oil and change the filter. Water in the oil will show up on the dipstick during this period as well. Oil that has water in it looks milky. Wouldn’t take too much stock in oil analysis at this point.
@Jody_Seal knows way more about marine engines and probably seen more marine engine issues than I ever will...