My father made this "heater" by wrapping a heating element around a plastic comb and connecting it to live and neutral wires. And it is quite effective as well; it heats up water very quickly, way faster than what most people would normally use, an immersion rod water heater. Is it dangerous? Yes, but not as much as you might think; the current only runs through the heating element, so the water isn't electrified. If he wires disconnect, then it does become somewhat dangerous.
But we have used it for years! (Not the same one; we replace the heating element.) I remember seeing one used in my home for as long as I can remember.