Water does not compress well so the engine would stall. When you are going slow at low rpm, you usually don't damage the engine. Just take the spark plugs out, then crank the car for few times. The water will be thrown out by the piston. Do this for about 3 -5 minutes to get much water out of the engine as possible. Also the throttle body may rust so you want to do this while you press the accelerator 1/4 way down. Reinstall the spark plug and car would start afterward. White smoke would puff out of exhaust for some time, but it should start. Once you got it started and warm up the engine, you might want to change the oil to get rest of the water out of the engine.The worst thing that could happen is when water gets into cylinder while you are driving at high speed. Water would not compress well. So the piston would dead stop and rotation force would be enough to break crankshaft into pieces.