TACKY!!!
Either just drink water or buck up and buy a drink. Otherwise stay home...
I got the impression the OP would be happy to buy a drink, if the restaurant served something she could have. I've never found a restaurant that carried diet caffeine-free Coke, or diet caffeine-free Mountain Dew. I would drink either of these. Or even caffeine-free iced tea. That's what we drink at home. That and Crystal Light.

My choices at a restaurant, if I'm going to buy a drink, are usually Diet Coke (or Pepsi) - with caffeine, or Sprite (or the pepsi equivalent) which has tons of sugar. Name your poison, in other words.
Interestingly, a restaurant is about the only place I can stand to drink the water. It comes very cold with lots of ice. At work we have bottled water, and although it IS very cold, I can't STAND it, and I have to add Crysal Light. I've tried drinking water at home too, even keeping it in the fridge, but...yuck.
Anyway, the OP doesn't even like the water in a restaurant. I wouldn't have thought about bringing in Crystal Light, but in that case, I don't see why not. She's not using it to avoid paying for a drink.
I don't know why restaurants can't just offer something with no sugar or caffeine. Then maybe many of the people now ordering water WOULD pay for a drink.