The Westsail Manual instructs a vented loop to be located high in the cabinet outboard of the head. There are 2 outboard and the outermost allows for the vented loop to be mounted higher above the waterline. I would think a loop in the inner cabinet would find itself below the waterline while the boat is heeling.

Carl, No, you aren't the only one with a head below water level.  I have a Groco EB head and originally installed both the discharge and intake loops in the void behind the outboard bulkhead, right under the deck level.  When it came time to "commission" the head I couldn't get any suction on the intake line.   After repeated experimentation, with the head in the boat and up on the dock trying to draw water from a bucket, I still couldn't get any suction with a vented loop.  I finally called Groco and they told me that I couldn't use a vented loop on the intake side!   It makes me nervous to have the line "unprotected" where a leak/failure could flood the boat, so I put an in-line valve on the intake line just inside the sink cabinet.  It's more secure, but I still haven't used the head!   Tom Koehl