Huh? If you don't drink enough water, you get thirsty. You have an inability to feel thirst or you're drinking non-hydrating fluids, diagnosing dehydration is going to be really obvious from observation and symptom questioning. I don't understand why a doctor would need to ask you about drinking water...
You know, someone whose brains are bigger than their ego would know when they've made a mistake and back off. Only a fool doubles down when they're wrong.