Re: 2 questions
Actually, all scales are different. It is difficult to know which is the most accurate...there are always variations by a few pounds. It is not beneficial to compare your weight among different scales.
I would be more concerned with the weight trend (i.e is your weight increasing, decreasing or remaining stable?) rather than the actual number. For recording your weight, it is most important that you are consistent with ONE scale. You should record your weight using the SAME scale in the same location each time. You should also weigh yourself at the same time of day each time and preferably without clothes on. Most people weigh themselves in the morning prior to eating anything. Weighing yourself everyday is also not usually beneficial as your weight fluxuates daily and it can drive you insane. Once a week or every two weeks is fine.
If you go to the doctor, and your weight is different from what you see on your home scale, don't worry about it. Just look at how your weight has changed from the previous time you were weighed on the doctor's scale.
As for water - nope water itself does not help with weight loss. It does not have magical fat flushing abilities. Some people claim that drinking lots of water helps them to feel full, and therefore eat less, but I have never found that to be true. For me, it just means more time running to the bathroom.
HOWEVER, water can be useful for weight loss IF you are replacing calorie-containing beverages with water. For example, if you are someone that drinks lots of pop, fruit juice, fruit drinks, sweetened iced tea, fruitopia, gatorade, etc, and you replace all of that with water (or any other calorie-free beverage), then that will help with weight loss. You will be elminating hundreds of calories from your diet.
As for how long it will take you to lose 40 lbs? Well, that depends on many different factors - the amount of calories you are cutting, your activity level, your metabolic rate. Everyone loses at a different rate. 1-2 pounds per week is common for a lot of people, although as a 17-yr old male it may be much quicker for you.
It could anywhere from 6 months to a year...or more.
Of course, it also depends on how often you cheat and fall off of your plan!!