The only drinks that tend have a net dehydrating effect on the body, however, are alcoholic.
That's what doctors have told me. And as I've gotten older, I'm having a tendency to dehydration, not unusual with aging. Once, during a routine doctor's visit, I was immediately diagnosed as so dehydrated that I had to have an IV drip for an hour, right there in the office!
So dehydration is a chronic problem with me, and I've taken to drinking water all day long. There's a water glass right here next to this keyboard at this moment. I hate it, but what are you gonna do? And I watch the color of my urine (I hope not TMI), and if it darkens (concentrating) I immediately start downing even more water like a fish.
If I have an alcoholic drink at a bar or restaurant I request a tall glass of water with it, which I finish along with the drink. In fact, bartenders who know us just automatically serve it to me. Because I know alcohol can dehydrate me, as you say.
I avoid coffee because I've been diagnosed as being hypersensitive to caffeine, almost toxic to me. I avoid Coca Cola & Pepsi for the same reason. I do drink some tea, but not other caffeine products.
Caffeine may be beneficial to some, but not to me. I watch my health and my husband's like a hawk. At our age we must do that.