If you're referring to water that has accumulated in your car's gas tank, you can buy a product called DryGas. Drygas added to your tank will absorb the water and allow your car to run smoother.
1 Additional Answer
If this is water in the tank of your car (mixed in with gas), one thing to try is a product called Heet. It is supossed to dry up the water. You put it right in the gas tank, wait a few minutes then start up the car.