He's wrong and loud.
Waiting for thirst in order to drink water is
like saying "I only buy fuel when the car stops moving."
By the time you feel thirsty, your body has already lost 2-3% of its water.
Your energy will drop. Your brain slows etc.
So how much water should you drink?:
Drinking small amounts throughout the day is not dangerous. What is dangerous is downing three litres at once because you haven't had water since morning.
Small drinks throughout the day. That's the koko. If you're urinating every 2 to 4 hours and your urine is light-colored, not white or completely clear, you're doing well.
Carry a bottle. Sip small. Don't wait for your body to beg for water.