When you lose the first ten lbs, how much is water weight?

Impossible to know. It's difficult to know what percentage of the pounds you're losing are water weight. If you're using a sweat suit, you could try collecting the sweat and other fluids you excrete after a workout and measure it that way (although it wouldn't be too fun). There are scales also that measure body fat and water percentage that you can get. Remember that losing water weight is always temporary!