I just built one last week. and instead of running it off the boats presure system , it just runs off a 12 volt and pumps salt water
...a stand alone system..
Took an old pump that the preasure switch had gone bad, attached a 2 foot line to the pump for suction side and a garden hose to the other side, tied a line to the pump and set it over the side.. it has a small fire-hose nozel on it and will shoot about 20 to 30 feet.
I plan on refining it so the pump is in a bucket with the suction hose going through a throu-hull in the bucket, safe-guarding the pump in the bucket..
side of it is just a plug
to conect it to the 12 volt system on the boat..
My understanding is that most of these pumps are designed to push and not to suck, so I've set it up so sucking water
is close to the sorce.