Shower water egress question & questions

Starting the plumbing system for the 34' Marinette houseboat. Trying to keep plumbing system basic and simple, but a few questions: The galley sink drain is to run out through a "Y" valve giving me a choice of draining out to gray tank or out the thru hull. Can I "T" in my shower sump drain ABOVE the "Y" valve to give me shower water egress through that same hose? I hope the pictures will help a little: http://s1216.photobucket.com/albums...ater system/?action=view&current=a818cb37.jpg
Notice that my one and only thru-hull is right about AT the water line, however, the Maeleron thru-hull is attached to a 90 deg. elbow that extends ABOVE the water line as depicted: http://s1216.photobucket.com/albums...ater system/?action=view&current=7580817d.jpg This is where my sink and shower sump is to drain. Here is the rest of the pictures and concerns and questions about my water system: http://s1216.photobucket.com/albums/dd377/projecthouseboat/Plumbing water system/
for example were should the water heater by-pass valve go: http://s1216.photobucket.com/albums...ater system/?action=view&current=7c62a064.jpg
Also, I want to install a manual hand pump in the galley to pull water from my freshwater tank. Where do I install that? Where on the actual tank, and is that a different water line than the electric/on-demand water pump system?

 

Usually a simple by pass valve allows the fresh water pump to be either the manual or electric , sucking from the same tank port.

 

There should be a one way valve to prevent city water from back filling the water tank. Is that in what you call a "distribution manifold'? Adding drains together have to take into account the total flow. If you connect the shower to the sink, it may back flow out of the sink drain. The plumbing doesn't need to be so complicated. The easiest would be to drain all the gray water into the gray water holding tank and have the pump connected to the tank drain.

 

Marine Plumbing is not as simple as you think.

Best to pick up a book like Nigel Calders ,Boat Owners Mechanical and Electrical Manual for reference and to avoid common mistakes like poor grey, black water tank venting . Several other books around...check at your library