I rededicated the head seacock to be supply for the reverse cycle AC pump (thank you Lord, after 32 years of heat in Texas, for reverse cycle AC at the dock at night). So I have just been pumping freshwater into the sink, which drains into the head. someone told me freshwater was better for the holding tank, but I am now on a fresh water lake anyway.
I think I can do a "T" off the seacock, with a backflow preventer and maybe a ball valve to open if I need to go back to raw water for the head. I was concerned that the pump for the AC would suck air backwards through the supply line to the head otherwise (does that make sense?)