Quoting Geek's Girl:" I know! But is it okay to not drink that much?"
Yes! After I had my first I never listened again! Ha ha ha.
About 1 1/2 hours before my ultrasound, I would fully drain my bladder. Once it hit an hour before, I'd chug my water. Never had an issue with it not being enough. Also with me being a lot over weight, it never caused issues. Also, if you do drink it all, it can cause your bladder to be too full. Happened to me, I drank what they said, and my bladder was too full! They expected me to go and pee out only a little. Also, if the appointment before you happens to run late, you'll be in even more pain!