Nope, not normal. But given your starting weight, definitely possible. It IS mostly water weight - see the sticky at the top of the page on ketosis. Long story short, you lose lots the first 2 weeks because when you cut carbs, your body first burns up the stored glycogen in your liver. Glycogen gets stored with water, so burning the glycogen releases the water. After the glycogen is gone, THEN you start to burn fat.
You'll probably have a good second week too, then settle into a pattern of less loss each week. But you will probably lose more in a week than I ever did - I'd guess you'd be in the 2-3 pound a week range for a while.
Good luck - you can do it!
Lost the first 100 with Medifast, now working on the last 25