On the few times I've flown Spirit (water/soda charge), I've brought (or tried to remember to bring!) an empty water bottle to fill after security. If this ever became standard, I might also throw some flavoring packets in my carry-on to add some variety. Or just pay the $2 when I got thirsty.

You could also freeze your favorite beverage and take it through security. Unless it's changed, the TSA does not consider frozen beverages to be liquids. That would be good for a long flight, I suppose. I have thrown frozen water bottles into a mini-cooler bag when I've also brought some perishable food with me for the family (another absurd problem with skimpy domestic service). I haven't been bothered by the TSA for this. They did bother me once with a gel-pak, so that's why I switched to the water bottle.