I try and aim for at least the 6 cups of fluid a day suggested in the booklet. This isn't always straight water, though - I do love my herbal tea. It's hard during the colder months of the year - I sometimes have to force myself to drink as much, as I don't get thirsty very often (and, of course, when you are, you're already pretty dehydrated!). I have had some good losses on the weeks where I've been conscientious with my water intake, and I always make a note of drinks in my food diary so I can keep track.
That app sounds a brilliant idea, Ponytales!