systems need an antisyphon loop in the exhaust to prevent seawater coming back down the exhaust and flooding your engine through an open exhaust valve. You then try to start it and it "hydraulics" ie full of water and busts your pistons
The fresh water cooled engine will have a salt water heat exchanger
to cool the circulating fresh water. The saltwater will exit the heat exchanger and be injected into your exhaust manifold and expelled out the exhauist pipe...the water you see out the back of your boat.
Any device that has plumbed in saltwater connections below (or potentially below) the waterline eg salt
water flushed head
, needs an antisyphon loop either to prevent flooding the device (engine) or your boat !
An d the loops should have one way valves at the top...but check 'em regulalrly to be sure they work..they can get stuck...
My advice ?.....Buy Nigel Calder's excellent book "Boatowners mechanical and Electrical
Manual"...it will answer almost any question you have..