There are two materials that are commonly accepted as appropriate for use as heating elemenents in potable water, copper and incoloy 800. The advantage of copper is that it has better heat transfer characteristics, and it is lower cost. So with the copper element, more heat energy can be output by a given design (higher watt density) without over heating the element, as it transfer more efficiently to the water, keeping the sheath temp low at even higher watt densities. In comparison, incoloy 800 has a significantly higher melting point and oxidation temp. It is also generally more corrosion resistant. The practical benefit of of using incoloy in potable water beyond corrosion resistance, is that because of the material's ability to withstand higher temperatures, when scaling occurs and insulates the elements, copper may melt or break down (fail), whereas incoloy can actually dry out the scale, causing it to crack and fall off the element. So you will still get scale buildup, but as it insulates the elements the incoloy is likely to last much longer.