She didn't really need another water - certainly not an 'expensive' kind, as nice as the gesture had been. Of course, there's the possibility it's meant mostly as a joke, but... she appreciates the company. And the puns.

"What did one coaster say to the other?" And reaching across, she grabs Sans' drink and holds it up at the bar tender, signaling for another.

"This drink's on me."

Grinning. She's proud of that one.