The loss of Water Intake History is usually associated with an update on your device, i.e. following the update you can't access the history.
The most common cause of this loss of Water Intake History is that there is a problem with iCloud sync. Here are some things you can try to fix it. Please note these fixes are only possible if you use iCloud.
1. Sometimes there is a delay in your device retrieving data from iCloud. This can be caused for a number of reasons. The solution is simply to wait for a few minutes.
2. If the data doesn't appear after waiting you can try turning the iCloud setting for WaterMinder off and then on again.
a) Go to Settings on your device and tap iCloud.
b) Tap iCloud Drive.
c) Scroll down to WaterMinder.
d) Toggle WaterMinder off, leave for a few seconds, and then toggle it on again.
e) Go back to WaterMinder to check if your history is now showing.