I know I panicked also as I have always had a small bladder. I even tried practicing weeks before to try and train my bladder. I did drink that much water and couldn't sit down in the waiting room. Finally my turn came and to get on the bed was just aweful. She put the scan on my belly and not more than 2 minutes later she said I am going to have to ask you to empty your bladder before we continue. I said to her, you have made my day! The scan went along just fine without my bladder being full, and they got all the pics they needed to so all is good.
I would just say drink enough till your comfortable and leave it at that. Bugga drinking all that fluid and holding it, I think with todays technology they could find something easier than having to do that!

Good luck and enjoy meeting your buba again!
P.S. Ask for pics of the gender if you want to find out, i wish I did!