I have worked with NaOH before, and I think the amount required would probably be more than a (US tank-style) water heater and less than a typical above-ground

I have worked with NaOH before, and I think the amount required would probably be more than a (US tank-style) water heater and less than a typical above-ground swimming pool. I think you would need some of the tables typically found in chemical engineering manuals to work out the exact amount. You would need the heat capacities for 50% and 30% (wt%) aqueous NaOH, mass fractions, enthalpy of dilution, and maybe activity coefficients. It sounds like a homework problem for a ChemE student.