Water in the gas tank is more than annoying. The air and fuel in the combustion chamber create an explosion which is essential for cranking the crankshaft and making the engine runnable. So, if water enters the combustion chamber instead of fuel, it will make the explosion weak and not allow the spark plug to generate enough spark. Water in the gas tank is an issue that might happen due to a leaking tank cap, or due to a bad tank breathing. Emptying the tank, replacing the fuel filter, and then refilling would be the solution. Consult a technician for a better recommendation.