You have to manually enter it but the app makes it as easy as possible. For instance, you can set custom water container sizes. I have a 24oz water bottle and have it set up so that I just have to launch the app of my phone and tap the 24oz button. I can also tap the watch complication to launch the app on my watch and track my water intake there.
I am a true believer that being hydrated is important to good mental and physical health. That being said, the vast majority of us don't take in nearly enough of it. WaterMinder makes the user cognizant of the amount of water he/she is drinking.