Guests constantly shut the shower off at the switch, either mistaking it for a light switch or being from a household that operates the shutoff every time they shower.

This is surprisingly effective.

The last one broke and left us without a shower for a couple of days. Apparently these are cutoff switches not designed for frequent use. The old one was probably defective from the start but one time too many did it in.
@zip
That seems .. odd. If it was an emergency isolator, there'd be no need for it to be in the bathroom - places rewired in the last 25 years or so tend to have push switches high up on the wall just outside (or above the door.) To me a pull cord implies regular on/off operation - it's a cord because it's in the bathroom, and it's in there to be close to the thing it controls for regular use??
@zip
Having said that, that design of pull cord switch fitting is the one I associate with emergency alarm cords e.g in accessible toilets. The ones in our bathrooms here don't look like that, but maybe ours are just out of date??
@srtcd424 @zip Those ones are smaller because they don't have to accept 6mm (or larger) cable and carry 45A.
@kim @zip ah, that makes sense, yeah. Electrically heated showers are not something I've ever encountered in any of our houses - you take you chances with whatever hot water is or isn't in the tank, instead :)
@srtcd424 @zip A far superior approach, IMHO.
@kim I dunno, I've had enough accidental cold showers that I wouldn't say no to some sort of automatic electrical top-up heating system :)
@kim (at some point I will get around to rewiring the system here for PDHW, which should mean I can then sensibly leave the HW 'on' most of the day, which should reduce the cold shower problem. Just need to find the round tuits!)