Diet soda is not bad for your healthy. This is because it has artificial sweeteners and other chemicals which are not harmful to human beings. Some of diet sodas are even fortified with vitamins and minerals. For more information about diet soda log in to http://www.mayoclinic.com/health/diet-soda/AN01732.
Diet soda has aspartame in it which is an artificial sweetener...it is not too great for your health(google it)...only some diet sodas have a healthier sweetener called sucralose which isnt bad for you
Soda is not the healthiest choice out there, but having a soda every now and then is okay, just be aware of the negative effects even diet sodas can have. Just think of the acid and loosing the enamel on your teeth.
Also, soda doesn't hydrate your body. It actually makes you thirstier and hungrier, which leads you to eat more. That leads you to weight gain, not the soda itself.