I believe the diuretic effect of caffeine in coffee is offset by the water in it. In other words, no hydration or dehydration. So you must still drink as much as ever on top of the coffee. Diet soda has less caffeine and more water so it's also not going to dehydrate you unless you do not drink as much water as you would normally need in addition to the soda.
Caffeine is a performance enhancing drug. Helps burn fat, too. A pretty safe and beneficial drug if used in moderation.