Gas stations do not water down their gasoline.
However moisture and water can get into the gas tanks through leaks or through the filling cap of the gas tank when the gasoline storage tanks are being filled.
Also you might get a bit of water in your own gas tank on rainy days or high humidity days as well.
If you do have some water in your gas tank and it's only a bit of water in the gas tank then you could pour a bottle of HEET fuel additive in the gas tank when you fill your gas tank up.
That will help remove small amounts of water in your gasoline and help your engine run better as a result.
I always add a bottle of HEET to my gasoline tank every second time I fill the gas tank up with gasoline and it does really help.
Adding a bottle of fuel injector cleaner also helps to clean the fuel injectors.
If the gasoline stations watered down their gasoline then the gasoline would not work properly in your vehicle and they would soon be out of business.
Gasoline stations make barely anything off of the gasoline and make most of their money on sales in the store.