High temperature high voltage electrical insulation under water
You won't find it easily, for a kettle element.

Things like araldite and heatshrink plastic tube will tolerate 100C (just), but unfortunately the inside business part of a kettle element runs far hotter than that, only cooling down to nearly 100C where it contacts the water. 

If you look at immersion elements, you will find a continuous metal tube extends out of the water, either extended from the same element tube, or welded to it, with ceramic insulator inside.

I'm not saying a suitable waterproof and heat resistant glue or plastic doesn't exist, just that it won't be as easy to find as looking in a normal electronics catalogue.