Milk, juice, pop, diet drinks, tea, coffee (regular and decafe) all count as fluid/water for the body. Each type has a limit regarding health. This SP article gives more on this topic. For diet drinks---no more than 32 ounces daily.
And, the diurectic effect of regular tea and coffee is "so minimal" that they also count as fluid. There is no need to drink an extra glass of water when you have tea or coffee. This is an old myth. www.sparkpeople.com/resource/nutrition_art
SP Registered Dietitian Becky