Water in your gas tank will cause havoc with your engine and cause engine troubles such as pinging and sluggish performance.
Your engine may surge, sputter and may even be hard to start or won't start at all.
Water can sometimes naturally get into the fuel tank through condensation and when it does it can get into your fuel system and then cause the engine to be hard to start or fail to start.
Water in the fuel doesn't allow the fuel to ignite properly or at all so your engine will have trouble starting or not start at all.
You may also get water droplets in the fuel where it could cause sputtering, stalling etc.
The water can eventually corrode the cylinders, fuel lines, fuel pump etc if you don't remove the water from the fuel.
If the water is just natural condensation in the fuel tank then adding some HEET fuel additive to the fuel tank can help remove the water from the fuel.
But if you have a gallon or more of water in the fuel tank then you need to drain the fuel tank completely and dry it out and then replace it with fresh fuel.
The fuel injectors can also be damaged eventually as a result of the water in the fuel tank.
So always use a locking gas cap or locking gas fuel door to keep someone from accessing your fuel tank and that prevents fuel theft as well.
I once had my nephew who was 3 at the time use a garden hose and filled my gas tank with water on my truck and he thought he was fueling the vehicle.
I was mad but now I look back at it and laugh because really he just didn't know that he was adding water to my fuel tank.
But now I have my gas caps locked so they cannot be accessed.