A free app for Android, by A Mobile Creatures.

Water Time is a handy reminder app to help you stay hydrated. If you don’t drink enough water, you may feel tired, have skin problems, be moody and even develop an illness. With this app you can keep a record of how much water you drink every day, so you can make sure you’re hydrated and don’t need to drink more than advised. 

With this app, you can also set up and customize your reminders, so you’ll never forget to drink water again. The nice thing about this app is that it’s very easy to use and understand.