How do I disconnect my boiler?

My city is on a mandatory boil advisory and while I know how to turn off the water supply, I’m concerned about burning out the element in the heater. We emptied it out to fill the bathtubs so we’d have water for flushing. I’ve never seen a circuit box like this (haven’t found anything on Google). In the box, if I remove the plastic cover where it says “On” I can see there’s an “Off” beneath it, but it doesn’t allow me to turn the cover upside down and jam it back in without possibly some kind of tool. Any help would be appreciated! Thank you!