Water and gasoline can't mix, and this is simply due to their varying densities. Gasoline is less dense (aka, it's lighter in form) than water. For that reason, gasoline and water can't mix and blend thoroughly into one. If they do come into contact with one another, water would sink to the bottom, while gasoline tries to float above the water. This is why the excess presence of water or moisture inside your gas tank is pretty bad. At first, it might not be seriously impactful, as the gasoline floating at the top of the gas-water mixture flows like normal. But, once fuel runs dry, the water that's sunk at the bottom of the gas tank will start flowing through the fuel lines and into the engine. At this stage, you'll start facing issues with starting up your engine.