Hi there, Gardenpixie! I don't know how to track net carbs on Fitday, but I do know how to track water. There is a Water, Tap entry in the database already. You can add that to your Food Log and track it like you'd track any other food. There are also Bottled Water entries. And, of course, you could make a Custom Food, if, for example, you like to add additives to your water (lemon, lime, Crystal Light or whatever).
There is also a "Water as an Ingredient" entry but personally I think that's more trouble than it's worth.
Anyway, just a few ways you can track water