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.