Rod_Knox, an engine running at, say, 3k RPM has tremendous stored kinetic energy in the moving cylinders. The water is not compressible and causes the cylinder to try to come to a sudden stop. Something has to give, eg, the connecting rod bends, or the valves are forced out, bending them. Or something else yields.
edit, to elaborate a bit more, the one cylinder that hits the water could possibly stop, but there are other cylinders moving and via the crank shaft and connecting rods are pushing that one cylinder with a lot of force, which can cause the connecting rod to bend or break.