A couple of points here. First of all, "stainless" means just that - it doesn't stain. There is no implicit corrosion resistance. If there was, they'd probably call it "rustless."

Everything corrodes in salt water. It's the nature of the beast. It's only a matter of time. Now, how much time is where it gets interesting. The industry actually has standards for these sorts of things, and it's actually a surprisingly short amount of time before even the best stainless steels start to show signs of corrosion when totally immersed in salt water. As in hours, not days.

But that's total immersion. You're not doing that. Still, how much time does it take to run a little tap water over the knife to rinse it off before going to bed? That's a valuable investment there. I think I'd find the two minutes to make sure it doesn't get ruined.