This is perfect for operating your 12v on your water heater. 

A momentary backlit switch can be fitted in reach of the driver’s seat (so they can turn it on when driving to preserve battery voltage) and its set to 30 minutes with each button press so again it minimises the risk of draining the battery if left on for long periods. It can also be switched off by pressing the switch again if needed. 

Simple to operate and doesn’t need any alarm reminders to switch it off.