no, this is wrestling. drinking water does NOT work.
two of my really good friends in high school were wrestlers, and i watched them do this before every single match...
1) eat very little, and only bread, or salad
2) dont drink much...in fact, as little as possible
3) the night before the weigh in, throw on as many layers as possible, ie - ski shit and stuff like that, get on a bike/treadmill, and just go at it...
4) bring a small power snack to eat once you've weighed in, like something with peanut butter and a gatorade
im surprised you dont already know how to do this...first year wrestling?