If you have water in your tank, or suspect that you do, you should take action immediately. If you do not, the water can get into your engine and begin rusting parts.
Before you begin treating your car as though it has water in its tank, you should make sure that is actually the problem first. The biggest problem water in your gas tank will give you is you will have a problem accelerating. You will press the gas pedal, and your car will simply not jump forward as it used to. You may have problems reaching highway speeds as you come down the ramp. If your car has not been experiencing other problems, this is probably a sign that water or a chemical has gotten in your gas tank. You may also see engine spluttering or great jumps in speed even when you are not pushing down the gas pedal extremely hard.
Here is the best option to fix your tank. First of all, you can simply empty out your tank via siphoning and fill it up again with new gas. Though this may seem a waste, it is worth it to avoid more expensive damages. You may even leave your tank empty for a time to make sure that all water has had the opportunity to evaporate.
Another option is gas treatment. This is a chemical solution that you will need to buy. If you tried the above treatment and it did not work, or if you simply are trying to improve your gas mileage, you can try this solution in your tank. After having had water in your tank, this solution will help your engine become more efficient (especially if you are using a cheaper gas that is not as healthy for your engine). The solution can help you by not only giving you better miles per gallon but also keep out any carbon deposits that are in the gas from reaching your engine and therefore hurting it. You can find this type of solution at an auto parts store or at your typical superstore. You should put the treatment in before you fill up your gas tank again so that the treatment is able to fully mix with the gas. You do not need to use it every time you fill up but every month or so is usually fine. If you are already using a high quality gas, you should not expect to see very many changes; however, using it can still protect your engine.