I think that you can get away with using the EEPROM. If you write the date and time once only when you turn the geyser on, you will only be writing a few times a day I would expect so the chip will outlast the plumbing by many years.
Note though that currently, you're turning the system on repeatedly. That's not ok for the EEPROM.
if you are still concerned though, take a look at a FRAM.