Welcome to FitDay and the forums!
You could track water a couple different ways, though probably simply in the Foods tab would be the easiest. You could also log it as a Journal entry, if you wanted (though I personally don't "journal", many people find it helpful).
Along with a good calorie deficit and daily exercise, a good amount of water intake is, IMHO, the third thing you need to do to maintain a good rate of weight loss.
47 M 5'8"
May 4, 2010...... 440? lbs. (Start FitDay Size 60 Jeans)
June 19, 2010.... 393 lbs. (First Weigh-in)
June 19, 2011.... 229 lbs. (164 lbs. gone in one year :-)
Current Weight... 185 lbs. (Size 36 Jeans)
Next Goal 169 lbs. (07/04/12)
Ultimate Goal 165 lbs. (12/31/12)
The best exercises for weight loss are Fork Putdowns and Table Pushaways.