This may not be exactly what you want, but this is what i've used:
There's an app called Healthy Habits that I love. In the free version you can track up to 3 habits. I set one habit as "Drink 6 cups of water per day." Then everytime I drank a cup, I would click the button and it kept track. It also would remind me if I was overdue.
The downside is that you can't enter "just drank 12 ounces" or "just drank 7 oz" or whatever. You have to set it in clickable increments. (In other words, every time you click it keeps track, but you wouldn't want to have to click for each ounce, so it works better if you define the amounts such as one cup per click.)