Gender ultrasound

So on Tuesday I am going to find out the sex of my baby. The ultrasound place informed me that I should be drinking half my weight in oz (take your weight ÷2 and that's how many of of water you should drink). However, that equals about a gallon of water every day! I drink maybe 4 or 5 16.5 oz water bottles a day. I usually only drink when I'm thirsty because over drinking isn't good for you either. Anyone else hear this rule?