"Clad aluminum" means the metal alloy is coated with a layer of pure (well, ok, 99.9% pure) aluminum, which is highly resistant to corrosion. MUCH more resistant than any aluminum alloy!
That's why "skin quality" aircraft aluminum sheet is nearly always Alclad.
Those of us who have been through aircraft corrosion control classes (yes, I am a licensed A&P mechanic...er..."technician") are well versed in all types of aluminum corrosion.
The water heater is likely made of 6063 aluminum alloy with a pure aluminum coating. 6063 is:
CHEMICAL COMPOSITION
Element % Present
Manganese (Mn) 0.0 - 0.10
Iron (Fe) 0.0 - 0.35
Magnesium (Mg) 0.45 - 0.90
Silicon (Si) 0.20 - 0.60
Zinc (Zn) 0.0 - 0.10
Titanium (Ti) 0.0 - 0.10
Chromium (Cr) 0.0 - 0.10
Copper (Cu) 0.0 - 0.10
Aluminium (Al) Balance
With magnesium and zinc (a trace of zinc) in the alloy, it is obvious why the clad coating!
