Just because water is fresh or "drinkable" does not mean it is not corrosive. All potable water systems have cathodic protection systems on their structures. Salt water is just more corrosive because the salt increases the corrosion rate. It has more to do with the pH and the "Langelier Index" than anything else. Very cheap insurance.

Yes it does....Corrosion is all about conductivity. Contrary to what you see in the movies, fresh, clean water does not conduct.

A boat that sits for a few hours per year in clean, fresh water will have zero corrosion issues for decades.

It all about salinity and cleanliness...