You can't drink water when you get a health certificate. Mainly because some laboratory tests need to be carried out on an empty stomach, such as color Doppler ultrasound examination of liver, gallbladder, pancreas and spleen, which requires fasting.
It is recommended to be on an empty stomach for at least 8~ 10 hour. Drinking water will affect the accuracy of the test results. After finishing these projects, you can drink water normally and eat some other food.