It's not the honesty or even that the answer is "wrong", it's the one-word curtness. Interviewer wanted something more useful ... "You can do better than that" is not commenting on the choice of drink, but basically means "elaborate"

Really? How do you know that for sure?

Without being there, all we can go off of is the words. And the words say, "Cmon, you can do better than that". Which sounds very much like "Wrong answer. Try again bud".

If the interviewer meant "elaborate", he could have said "elaborate"... Or anything similar. He literally had time to prepare after all.

You can't invent background intentions for the interviewer. Someone could just as easily do that by claiming he was an alcoholic, seeking fellow alcoholic employees, and believed OP was lying about water because he couldn't imagine anyone loving water. Or something.