When water gets inside your engine, it'll cause something known as hydro-locking. As you know, an engine functions by combusting a mixture of fuel and air. To maximize this burn, an engine will use the movement of its pistons to further compress the air and fuel mixture. Seeing that air is gaseous and fuel is mostly sprayed in a vapor form, it can be compressed fairly easily. However, water can't be easily compressed. Should water get into the combustion chamber, the engine might seize and stall, as your pistons are now facing a mostly incompressible liquid. Unfortunately, some engines might not be so lucky. The excess strain and force could easily bend connecting rods and puncture a hole in your engine block. Thus, causing complete engine failure.