I have been drafting this thought a few times; but is my suspicion right that the weapons (except the knife, of course) also corrode after having been in water? or do they only break after a certain amount of rounds / reloads? Right now I piss myself off each time I happen to fall into a river on my way to a mission, scared that my weapon will start messing up in a multiple-target shoot-out, cause there's never a arms dealer nearby when you need one :p
I don't think water speeds it up any.
Like when I first got my hands on the USAS12 shotgun from the armory, I got it, the Eagle and went off to do that 'mobile radio' mission in the second part of the big world map.
Corrosion happens from [I think, my idea on it] running around with the weapon in your hands and also from shooting it. If you don't draw it, it won't corrode, that's how I think it works.
Also some weapons are just more likely to jam, like the g3 rifle that's all over the place, the AK, and the pump shotgun and ofcourse pistols. Sniper rifles, besides the 1903 sniper, rarely jam, and I've never had my M79 jam on me.
Once you find a golden ak [i've found 3 out of the 8. Bastards are hard to find] put it in your weapon safe at a safe house, they never jam.