If you get water in your gas tank the water will get into your fuel system and then when the water is picked up by your fuel pump and sent through your fuel lines and into your fuel injectors the engine will start running rough, engine will stall and/or the engine will fail to start or run.
The engine cannot run on water and even tiny water droplets can cause an engine to stall or run rough or be hard to start and stay running.
If you have just a tiny bit of water such as from rainwater or moisture in your gas tank then using some HEET fuel additive can remove that water.
But if you have more than a pint of water or a liter of water in your gas tank you need to drop the gas tank and drain it out.
Then after you get the water out of the gas tank you can reinstall the gas tank and fill it back up with gasoline.
You need to also get the water out of the gas lines as well.
Water in gas tank will cause a car to stall, run rough, surge, idle rough, spit and sputter or even completely cause the engine to die or not even start up at all.