How are electric water heating elements isolated?
The element is not made of copper (the resistance would way too low).
Normally it'd be nichrome (nickel+chromium) and then enclosed.

This is a straight quote from wikipedia 


  Tubular (sheathed) elements normally comprise a fine coil of nichrome
  (NiCr) resistance heating alloy wire, that is located in a metallic
  tube (of stainless steel alloys, such as Incoloy, or copper) and
  insulated by magnesium oxide powder.