Any idea as to the difference in power consumption between making
potable water from the ocean and making potable water from gray water?

On a proper boat with modern charging systems running a watermaker direct works
fine. My boat had a watermaker installed when I bought her/ No grey water
tankage. The 34XL has a water maker. No grey water tankage. The world cruising
Hughes 38 at my yard doesn't carry grey water, nor does the Valiant 40 that
heads south each November.
You know NOTHING, Jane! Buy a real boat and then open your wrinkled yap!

Bwahahahaha!

RB