I have often contemplated getting an immersion heater element and rigging it up so that if I have a full tank of water I can go out in the morning with the water preheated in a well lagged tank.
Set the thermostat, link it to a timer over night and see what happens.
I have absolutely no idea if that would work or not, the element might even contaminate the purity of the water and over the course of a few hours the water would cool down, even in a well lagged tank, but might be worth a try...mightn't it?


Even if it is useless, the cost in trying it out is minimal.
Ian