A small amount of water in the gas tank is common, especially if your gas tank is usually half full or less. This is due to condensation, and is really nothing to worry about.
If you have LOTS of water is your tank, you will know it because your engine will buck, stall, and otherwise not run correctly, since water cannot be burned in the engine as a fuel (despite the many claims to the contrary). It is "possible" to get this much water in your tank, but it happens very rarely, and only under abnormal circumstances. It would be very unusual for you to have a harmful amount of water in your tank. Not impossible, but unlikely.
If you think you have more than a tiny amount of water in your tank, you can purchase a bottle of Dri-Gas, or a similar product (there are many), at an auto parts store, or even at your local grocery store. These products will absorb any small amounts of water in the tank and eliminate them. I usually use such a product once or twice a year, especially right before winter sets in, to avoid fuel line freezing. It's probably unnecessary, but I once had a frozen fuel line, and I don't want to have one again.
Is there a reason you suspect water in your gas tank?