I leave a carbon steel #8 in my backpack as a "food knife" as well as something that I could give away if anyone ever REALLY needed a knife. It gets damp from rain, riding through puddles, etc, and it's held up fine so far.
Don't put it away soaked, and try to let a natural patina build up because that way you won't rust (cut some acidic foods with it and then leave it for a couple minutes).
I think that once you've got a patina on there, it's more a matter of just keeping it dry(ish).