The worse thing that will happen is that they tell you that you didn't need to drink any water and you can just go empty out. My one at 8wks I didn't have to drink anything but the one at 12 weeks I did.
I would suggest go to the washroom an hour before and then drink about 20 oz of water. I think I worked it out to about a bottle and a half. But when you feel like you have to go STOP! The worse thing is when they tell you that you are too full and you can only go this much and hand you a tiny little cup. You want your bladder full but not ready to burst full.
You don't always have to do this. I had ultrasounds at 7 and 13 weeks and never had a full bladder. The only reason they sometimes have you do this is because it pushes everything up, making it easier to see the baby. In advanced pregnancy (which is when gender is determined) there is no need to push things up as the baby is clearly visible. If you weren't told it, don't worry about it. You're more than far enough that it's not needed.
no you should have been told to drink water, you need to drink about 1 ltr around an hour and a half before your app and hold it in.... fun :)