Morning ladies!

I have an ultrasound this morning and I'm 34 weeks. They've told me to drink 4 large glasses of fluid an hour before. This is the first time I've had to drink anything for any of my ultrasounds, so I'm wondering, do I really have to bother? To get to my appointment I'll have to walk 20 minutes and catch a subway, needing to pee, with a baby bouncing on my bladder like a trampoline. I'd really rather not drink the water : ) Baby will be big enough to see without it, surely?