I'm quite lucky, my weigh in is 09:00 Sat morning.
I should imagine you'd be fine as long as you don't drink for 2 hours before hand. That should be plently of time for the water to pass through your system.
I don't alter my water on weigh in day. That way it's similar every week. Water is important for losing weight so I don't want to reduce my long term weight loss for the sake of a short term weigh in - it seems counterproductive.
If you have the same before every weigh in it's always going to be with the same amount in. Don't make yourself dehydrated just for a couple of ounces.
My weigh in is at 11am on a sunday, and I know it's quite naughty but I dont have anything. But as soon as im weighed I eat a bar and have a glass of water x