stainless steel water heater water quality
Austenitic stainless steel ( non-magnetic) is unlikely to crack with chloride less than 100 ppm. Ferritic stainless ( magnetic - most auto exhaust pipes) will not crack in chlorides. Typical potable water is not corrosive to stainless or copper so galvanic corrosion would only be a problem in you also have plain steel. Although I believe "regular" water heaters are the most practical.