Stinky it takes years of experience and thousands of hours of on the job training to even come close to understanding plumbing code and sizing. A 30 second Google search just doesn't cut it, sorry.
Go back and read what I said again. I said it will take very little water to cool the magnetrons(remove the heat generated by the use of the magnetron). I implied that the heat produced by the use of the magnetron can be quickly removed with very little water. Just like a microwave, a small fan keeps them cool. Very little water will be needed to keep the WDRP magnetron cool. So thinking this heat generated will have any significance on the overall ouput temperature of the water is incorrect IMO.