U.S. here, but I was told to drink at least half my weight in ounces of water two days before any scan. It helps boost your amniotic fluid and makes it clearer for better imaging. More fluid means more space for baby to move. On the day of, you are supposed to drink as much water as you can and maintain a full bladder if possible.
Recommendations can definitely vary though. I was also told no caffeine in the days leading up to the scan. It can make the fluid appear cloudy.