1990 2700 scr plumbing

[Derekmcl]

Hey Folks!!!

wondering if I could get the plumbing in my boat explained.
I have a stern mounted shower, head, shower and sink in the cabin.

I was told the head pulls water straight from the lake, but I am unsure of that.
I do see an inlet ( brass fitting) on the bottom of the boat.

I have not been able to make the showers work. Any help on this would be great