When a small amount of water gets in your fuel tank, it can cause the engine to runroughly and stall. A larger amount of water would make teh engine stall and not restart. In addition, water can corrode fuel system parts (such as electric fuel pumps).
If you have obtained a fuel sample from the vehicle and can see any water droplets in the fuel sample, you might try a bottle of dry gas; this disperses teh water droplets so that they do not all get sucked into the pickup screen at once. If there is a larger amount of water in the fuel, you will likely have to remove and drain teh fuel tank to get it out.
This is an extremely unusual thing to happen these days; gas stations now have to have double walled encased fuel tanks with monitoring alarm systems to prevent water from getting into them. Years ago you used to see water get into fuel when a gas station tank developed a leak, but I have not seen this happen for many many years.
If you suspect water is in your fuel, any repair shop can draw a fuel sample from the engine with a fuel injection pressure gauge set to confirm this diagnosis. If they DO find water in your fuel, you may want to invest in a locking gas cap.
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