I had a bowl of cereal before leaving for my appointment and drank about half a bottle of water on the way. My scan came out beautifully and I didn't have to pee any. I don't think it really mattered how much I drank, though. I think it matters more about how much you drink in the days leading up to it, since that increases the amniotic fluid volume which gives LO more space to spread out so they can see everything. That's just a theory, though, based on what I've seen said about 4D scans.