[Arkanum]

I don't think any current flow in the water is deliberate. The heating comes from the resistive heating of the coil. The issue (i think) is the coil being uninsulated, allowing some current to leak out through the water, hence the tingling sensation others have mentioned.

[londons_explore]

My bad... the video features a design with a coil. There are others which use a grid and the waters conductivity.