Well it wouldn't be a very efficient
pump, and I don't think it very likely the cause, but every other down stroke of the piston with the stuck valve would pull water
into the manifold and cylinder, not all of which would be expelled by the subsequent upstroke, possibly yielding a net positive gain of water in the manifold, allowing the normally operating cylinders to be flooded on shutdown. Almost all pumps with valves operate with some level of 'leakiness' in the valves...
Overcranking with an inboard/outboard is not normally an issue; the water generally just runs out the outdrive, or in the case of some high performance applications with through-the-transom exhausts, directly out of the boat.
Generally speaking, the parts
that let the cooling
water out, and keep seawater from coming back in are pretty reliable, but if they get stuck closed they can let water back up into the engine
, or if they get stuck open can let seawater in in adverse conditions.
In certain applications, the engine
can be mounted too close to the water line, making having an issue along those lines more probable.
At 10 years, the manifolds and risers can be shot even with low hours, in my experience Volvos' cast iron manifolds are a bit worse for longevity than Mercruisers', but it's really more dependent on the operating and storage