God, as purportedly quoted by the author of Deuteronomy, interprets this as putting God to the test (this, if anything, is the dubious bit; the story in Exodus sounds as if their problem was thirst more than it was doubt)

They doubted God's ability to provide them with water.

Now I'm imagining it a bit like a long journey in a car, with Israel the nation being like a very whiny child:

"Are we there yet?"

"We'll get there when we get there."

...

"Are we THERE yet?"

"No."

...

"Are we there yet?"

"No, we didn't get there in the last ten seconds."

...

"I'm thirsty."

"Here, have some water, and if you can't sit quietly for a few minutes, I'll have you wandering the desert for forty years, okay?"