You have two options when installing WaterMinder app on your Apple Watch: if you don't have it on your iPhone you can download it just for your watch; or you can sync the app that is on your iPhone to your watch.
1. Tap the Watch app on your iPhone.
2. Browse to apps and tap on the Search tab.
3. Type waterminder and then download the app.
If you already have WaterMinder on your phone and want to sync it to your Apple Watch, you should do the following:
1. Tap the Watch App on your iPhone.
2. Scroll down the list of apps and tap on WaterMinder.
3. Make sure "Show App on Apple Watch" is toggled on