A: call it "raw water cooled" (instead of salt water cooled), to distinguish it from a "closed loop/heat exchanger" system. You see, if you are in a fresh water lake, your "salt water" system would be using fresh water. So, to say "raw water" means that everyone understands you are referring to a system that uses cooling water that comes from outside the boat, be it salt of fresh. The water that surrounds a the engine block in a closed loop/heat exchanger system is commonly referred to as "fresh water".
The main problem with raw water
cooling in a salt water environment
is that the salt will begin to leach out of the water at about 160deg F. Most marine
motors, especially diesel
ones, want to run up around 180-190deg F. If you try and use salt water as a coolant
at that temp the water channels inside the motor
will eventually get clogged with salt/mineral deposits. The other choice is to run the motor
at a lower temp, but with diesels, you invite all manner of other issues related to running too cold.
There are thousands of boats that are sailed in fresh water and are using raw water as a coolant
without problems, except one: You will not be able to add any anti-freeze to your block, which these days has lots of other good additives that help keep things lubricated etc inside the water jacket of the motor. Not to mention what happens if you live in a cold climate.