Thanks for the extremely useful information.
I have yet to decide on the source of heat but on the plumbing side it is clear that it will be a self-contained shower pan that is drained by its dedicated pump, probably into the head sink outlet (which already discharges greywater through its own through-hull).
We've had the rail in the water on a regular basis. I was wondering where the water was coming in from!
Looks like I,ve got a few adjustments to make.