the real problems happen if the engine is running especially if it is running at speed. water, unlike air, does not compress. If a cylinder fills with water and the engine is running when it tries to compress that water and cant something else has to give, usually its the rods.
Consider this, water evaporates at 100C, pistons, valves and heads are around 260C(combustion is much higher) when water evaporates it expands 1600 times in volume. The pistons most likely never get a chance to compress anything because they are already going backwards. Those of you who are trained fire fighters would most likely confirm this theory.