The water will mix with the fuel and create an emulsion. This emulsion will cause the engine to run rough and may eventually stop running altogether. The best way to prevent this is to regularly check your fuel tank for water and remove any you find.

Water gets into the fuel tank and it will mix with the gasoline. The water can cause corrosion in the fuel system and damage to the engine. Water is heavier than gasoline and it will sink to the bottom of the tank. The water can cause a gel-like substance to form that will block the fuel filter. The water can also corrode metal parts in the fuel system and cause rust. If enough water accumulates, it can flood the engine and stop it from running.