An ounce of prevention is worth a pound of cure.
It isn't necessarily a bad idea to lay off the fluids (especially coffee/soda/tea's or anything with caffeine in it) during the morning before your required surgery.
If you can't function without caffeine, then drink it concentrated so as to not overload your bladder.
Now, I'm a boy, and nature calls often enough for me as well, but I never had a surgery where I had to break scrub because of a bathroom break.
And usually, if they're long surgeries (at least at Mizzou), the attending will take a break in the middle, then the fellow, and so forth.
The "good student" should stay that whole time. But if they invite you to break scrub, and you need to go, (or to eat, as I had to), do it. It may be your only chance. They won't be mad, if they extend to invitation to you.
Good luck. Surgery is a rough one...and I'm glad I'm done with it.