Water and fuel don't mix, so it's never a good idea to put water into a car's gas tank. However, when water does get into a gas tank, either accidentally, or as the result of a malicious prank, the result is engine trouble.
Water can get into a gas tank even if you didn't put it there. Sometimes water gets in through a badly sealed cap on the tank, or by natural condensation on the inside of the gas tank
Water in the gas tank may lead to the car bucking, stalling or not starting at all, or starting the car could be difficult. This is because the water and fuel separate, especially when the car has been sitting for a while, and the water sinks to the bottom with the fuel on top. Water can also get into other parts of the engine causing rust and corrosion.
The method you use to get rid of water in your car's gas tank depends on how much got in there in the first place. In the case of only a little water, an additive that dries up water could be used. The additive combines with the water as it goes through the running engine. If there is a lot of water, a mechanic will have to remove the gas. This may include removing the fuel tank from the car and putting in new filters. The tank is then put back and is filled with clean gas.