Where could I find some good insight into designing the plumbing to get
water out of my shower drain to an underwater thru-hull, and staying
out of the shower? The plumbing in my Beneteau allows some water to
come back into the shower drain at the base of my head, enough that
when I heal the boat, it coats the bottom of the head floor.

Seems after the pump stops, the extra water in the hoses to the pump
slowly let out water, maybe only 4-6 ounces, but enough to fill the
bottom and come up a small amount above the drain.

I suspect its designed poorly and I could upgrade it myself. I noticed
for example the diaphram pump wasn't design to be connected to
underwater thru-hulls.

The current plumbing has a hose running up then down the wall to a thru
hull, in a loop,with an in-line filter and a diaphram pump at the top
of the loop. After I pump the water out of the head, even if I close
the thru hull, water runs back into the shower drain within a minute or
two. If I stick a cork in the bottom of the drain, the water gets
around it and forms a tiny pudle in the middle of the head anyhow.

Dan