Ok - quick review - there are two separate systems. The fresh water
system is a closed system. For thoroughness just make sure the fresh water system is full.
Then as Pat has said focus on the raw water
system. Water is drawn in to an impeller, it is "pushed" forward through a heat exchanger where the cool raw water
runs through tubes and takes heat from the fresh water across the metal surface of the tubes.
There it flows into the exhaust at a mixing point finally being ejected at the stern.
You condensed the steam vapor in a metal bowl and confirmed that it is salty water.
Water is converted to visible steam through too much heat or more correctly a large enough difference in ambient temperature and the vapor temperature. i.e. hot saturated air becomes visible steam at lower relative ambient temperatures. Just like a car exhaust in winter.
So you have visible steam because it is colder outside than you are used to seeing. Or there is less water hitting the exhaust mixer and less water with the same heat = higher vapor temperature @ same ambient temperature = visible steam.
The question of whether it is safe to operate your engine
with visible steam is whether there is plenty of water flowing out the exhaust. If there is insufficient water to cool the fresh water system, you will overheat and boil off the fresh water system and then you run the risk of overheating
You mention that you found debris in the heat exchanger - does this mean the impeller is damaged? In any case if there is "little" water in the exhaust you could have blockage in the heat exchanger that you cannot see. You may have to pull it and get it chemically cleaned - any radiator shop should be able to do this in a chem tank.