Hmm... I think that it sort of depends on the application; many boat's harnesses are non-tinned (engine and 12V) and you don't see huge problems with fires. Yes; there are some here and there but when you look around marinas the vast majority are older hulls that likely don't meet current (or any) ABYC codes. That's not to say that many boats don't have hidden problems in their electrical systems. I'm just pointing out that the incedence of fires relative to the number of craft is pretty low.
Wire that is not under a high load (like a bilge pump or anchor windlass) usually will function normally until the strands are failing due to corrosion of the core of the strands. This generally does not happen; the surface oxidizes and it takes much longer for the strands to fail. The bigger issue is corrosion at the point of attachment to a crimp connector; and the oxidation increases the resistance at the contact point (resulting in heat). Again; the current being delivered makes all the difference in how much heat is produced. Some low voltage/amperage devices will cease to operate (like cabin lights); while others like battery cables and anchor windlasses would get hot/smoke or catch fire due to the high resistance and high current.
I'm not saying that oxidation of the wire strands is a good or in any way desireable; I'm just saying that surface oxidation is not necessarily a death sentence for otherwise good/serviceable 12V (low amperage) applications.