This dubious device heats water by passing current directly through the water. It consists of two metal plates, one connected to line, the other neutral. 240 volts AC is applied between them, which not only heats the water but ensures the water can provide substantial electric shocks as well.
That's right - this is just two metal plates in a lump of plastic connected to the electrical supply. No current limiting, no fuse, nothing to prevent contact with live parts.
This first video shows the device itself and demonstrates it operating. A probe is used to see what voltage exists between the water and ground - the same voltage that would be applied to a person if they put their finger in the water with the heater.
Current is also measured using a filament lamp to limit the current to a moderate level.
Following the first video and the comments that resulted, this is take 2, where voltage and current are measured at the same time. An LED lamp is also used, with further explanations as to how dangerous this thing still is.https://www.youtube.com/watch?v=P-dmgYbhLSk
Obviously this dangerous item cannot be used safely, so it must be destroyed.
Pouring salt into the water is a sure way to increase the current significantly. The device fails, but the failure may not be what you expect.
While these things do heat the water, the metal plates react with it forming a nasty brown or black sludge. No idea what that is, but is certainly isn't something you should be drinking. Placing fingers or other body parts in the water will result in electric shocks, and leaving a device like this unattended will probably result in overheating and flames.
Buy and use at your own risk. Or just don't.