Also I have never seen anyone in the Philippines use a heating device for anything but cooking.
Electric water heating has a significant market share world wide, roughly 40%.
Afaik, electric heating of anything is incredibly inefficient compared to natural gas or propane. Thus this isn't making the world more energy efficient. This is for people who don't care about efficiency because they can afford the safey and lack of hassles of not using gas.
That's an strange way of measuring efficiency (yes I know you said "energy efficiency", I just consider that measurement somewhat irrelevant). If it has better safety and fewer hassles, that increases efficiency.
The fact is, for whatever reasons, 40% of the water heater market buys electric. Unless that changes, adding mining chips to that is essentially free mining.
Free for whom? Those who will spend outrageous amount to heat with electricity are receiving an insignificant amount of gain. You can give me 10 free pennies per month and I don't have the time to pick them up off the ground. They are not free for me to pick up.