I just had my first pregnancy ultrasound last week. My letter didn't say anything about drinking water. So I didn't. The midwife told me they don't practise it at that hospital, and they only ask u to go drink some water if baby is asleep as the motion of the bladder filling sometimes wakes them or if that doesn't, emptying it does.
Of course my baby was fast asleep and I had to go out and down two bottles of water. Lol.