Water has a higher density than gas. It doesn't belong in a gas tank. So, if gas and water are combined for some reason, the gas will float on the water. If a lot of water gets into the gas tank, this could cause serious issues with your car. For example, it might stall or just not start at all. This will happen because water will fall to the bottom of the engine. If the water gets into other parts of your car's engine, it can cause other problems as well.