The problem is, it's a loaded question which is phrased so that anyone who has consumed alcohol, no matter how small, is asked to implicate himself.
Based on the question, I think drinking water as an answer would be honest. However, saying I prefer not to answer that question (if one has consumed alcohol) would probably be the better response if one does not want to admit guilt.
Saying you prefer not to answer a question like "Have you had anything to drink to night?" would probably not go well for you. Whether you had been drinking or not, you are probably going to go through a battery of tests right there on the spot. Officer would likely do all he could to make you regret your comment before letting you go.
Besides, officers are smart enough to tell when an answer is deceptive. Saying "Yes, I've had some water." is just going to lead to the follow-up question of "Anything else? Besides water?" or even "Have you consumed any alcohol this evening?"
I do not think you are getting out of admitting you had alcohol (if you did) without lying to the officer in your scenario. Unless you pull the refusal to answer card which I addressed above.
Easiest answer is do not drink and drive. Seriously. It is so simple.