Why use stored water. Why not use lake water - that is how my SCARAB was rigged.

Flush water...Hold shower head over bowl and turn on shower.

My Baja Islander has a fixed potty in it with a large hose (guessing 1-1/2") with a filler cap on it like the gas cap next it and a smaller vent hose. Is that just to help fill it and then I carry the prizes out? Sorry for the stupid question but I really have no idea what I'm dealing with here.

almost all boats rigged that way from factory, the sea water that lays in the lines, head, tank stinks like hell in the heat, I convert all my boats to fresh water flush, keeps head/hoses cleaner, and in this case you do not need to add a thru hull