I'm not up on chemistry but yes, the white precipitation would be from salts in the salt water. And again, yes, it certainly could be sealing and protecting the bare metal underneath, so if it isn't creating a problem, letting it alone might be the best thing.
I understand that everything gets built to a price, but raw water engines....I just cringe that they are so popular. That's a mean thing to do to an engine, especially since those mineral buildups start at about 140F and engines simply are more efficient when running at 180-210F, which can't be done on raw salt water.
I guess no one really was thinking about "You know, this boat could last fifty years" when they spec'd raw water.
I never liked the thought of raw water cooled either. And now days I think the only raw water cooled engine you can buy is a Yanmar 1gm (only 8 hp). But after owning a direct sea water cooled engine, I like it. Maybe like it better than a closed cooling system (although a keel cooler would be the best system I think). A well built sea water cooled engine is much simpler, and if you get a leak in the cooling system, no big deal, you have an ocean full of cooling fluid to use. Also the system is much simpler- one less fluid pump to maintain, no worries on antifreeze/boil over fluids to maintain, no heat exchanger to deal with, and all those fewer parts means the engine is easier to work on and get to.
I think the good direct sea water cooled engines were actually more expensive to make. That is why most engines today are closed cooling systems- they can take a regular tractor engine and throw it in a boat, where as a direct sea water engine had to be carefully engineered using correct materials and zinc to protect the engine.
An engine may work best at 180 deg F, but I would rather run at 140 deg F in a boat. The cooler running engine makes the engine compartment cooler- less risk of fire, and the engine likes sucking in the cooler air.
With both boat diesel engine types lasting a very long time if maintained, either type should provide adequate service.