Waterminder

If you’re planning on running, exercising, or you just want to hydrate more, then Waterminder should be the first app on your Apple Watch.
Waterminder makes it easier to keep your body hydrated and to reach your daily and weekly goals. How do you know what your goal is, I hear you asking. Well, Waterminder does that for you, too.
When you first launch the app, it’ll ask you a few details about yourself, including your height and weight. Once you enter that in, Waterminder will tell you how much water you need to drink per day.
Afterward, the app will keep reminding you to drink water throughout the day. Once you’ve had water, you can enter the amount you drank, and you’ll see your progress bar go up.
The notifications can get a bit annoying, and you’ll receive them even seconds after you just enter the amount of water you drank, but they do make you remember you need to drink water.
Another great thing about Waterminder is that you actually feel you’re making progress. The app shows an animation of your body getting filled by the water you drink, which motivates you to keep drinking.