Re: jabsco pumpout , hows it work ?
The shutoff valve in hte bilge is a seacock. It is a safety mechanism that allows you to shut the valve should a hose rupture. Otherwise, without it, if a host ruptured all you'd have is a hole in the hull of your boat pouring water in and quickly sinking the boat. Yes, the seacock needs to be open to allow raw water to be pumped in to flush. IT IS A GOD HABIT TO SHUT THE SEACOCK WHEN LEAVING THE BOAT FOR ANY LENGTH OF TIME, SUCH AS OVERNIGHT. (You will need to remember to open it before using the head. It is ok to leave it open as long as you're on the boat).
The pump on the head is used to both pump raw water in and to push blackwater out to the holding tank. The lever on the toilet toggles between the 'Dry Flush" and "Wet Flush". Wet Flush draws water in from the through-hull/seacock. The Dry Flush pushes it out to the holding tank. You should only wet flush with the seacock open. Pump a handful of times in the wet flush, then pump a handfull of times in the dry flush setting. If the pump handle can be turned a quarter turn to lock it, then I would do so, this is another internal seacock on the head itself.
Here is a good link:
http://www.jabsco.com/files/29090_29120_toilets_0406_data_sheet.pdf
Operating Instructions start on Page 5.