Yes this would be a great app for the Ionic. Hopefully a developer could make one that also connects to your fitbit account to update the water intake there.
I agree. I often do not have my phone when I refill my water bottle and would love to be able to mark it on my watch rather than having to go back and get my phone.
Also think this would be much easier than a couple food log, via My Fitness Pal or otherwise. Just water. I just want to mark off water.