As far as using the water to add to your minimum weight, most design class racing won't permit that, and you'll get restriction weights. But if you're headed to a distance race, it's almost a must for equipment. As you say, I always get thirsty on the wire, and it's not exactly convenient to have to go down and rummage through stuff to find a drink.
A note of advice here. My crew has the smallest "backpack" possible (no extra storage space other than the fluid bladder). This keeps her from getting hung up while tacking. I have a larger one (storage area that is, we have similar fluid capacity - 70 oz) so I can keep a radio and emergency equipment on my person in case I fall off the back or break something. Not to mention it's a good place to put energy bars (crew has to get them out though)..
My 2 cents
Team Katastrophy