I haven't ever used it in salt water, but I have a little 857 with a bright orange handle that's been in my tacklebox for 7-8 years now and many fishing trips, and I will say that fresh water just doesn't seem to affect 420HC in the least, knife is still spotless and looks brand new.
I really like how easy it is to touch up the edge on 420HC out in the woods and water, have used a wet flat rock to do it, not sharpening really mind you because the edge wasn't dull, just used real light strokes to strop the edge basically and it was back to fish-stabbing sharp, lol.