Id just say the cold water.. but you have to drink water anyways for your scan. I never used anything with my 3 kids and found out each time (5 different scans in total). With DD2, we were able to see the sex but the scan guy was having problems getting other measurements because she was sleeping so he just told me to get up and go do a wee, so maybe if your bubs isnt cooperating you can suggest you do the same.. or just get up, jiggle your tummy a bit or something. That always got my babies moving too.

Best of luck!