Diet Soda is essentially just flavored seltzer water with more flavor than the drinks actually labelled as flavored seltzer.

A lot of people in this thread don't seem to understand that.

Most of the people touring horrible health risks have conflated diet sodas with sugary sodas, some have misunderstood some genuine studies on diet soda. The reason diet sodas correlate strongly with certain health conditions is primarily selection bias. Many diet soda drinkers also eat high-fat, high-sodium, US-style fast food diet, which is actually pretty hard in your body. Also Diet Soda's sweet taste can trigger heightened insulin production, because your body expects it to be full of sugar, and that heightened insulin can cause sugar cravings, and some people then consume more sugar satisfying that craving that would have been in a regular soda anyway.

Diet Soda is not a diuretic, it hydrates you just as well as tea, seltzer, or even a slightly smaller glass of plain ol water. There's some unanswered questions about the artificial sweeteners and their long term effects, but any of the really bad stuff they've found in actual studies has been at far higher concentration that a person would consume even if Diet Soda was their only fluid intake.