Per my and others' earlier posts, most likely it is the rubber end caps on the heat exchanger or the heat exchanger itself. But thinking about it more, how do you know it is salt water and not fresh water? A taste test would be difficult with antifreeze in the mix.
The only other place I can think of for water getting into the engine's sweet water/coolant circuit would be the heat exchanger in your hot water tank. If your other checks come up with nothing, try looking there. With the radiator cap at about 15 psi - 1 bar, your fresh water pump may be about 20-30 psi - 2-ish bars and could conceivable push some fresh water into the engine cooling heat exchanger. Maybe a long shot but worth checking if nothing else is wrong.
Good luck with it,
Craig SN68 Sangaris with your same engine only with a Perkins label. You're not even close to needing a new engine!