The 100R&R is specifically made with wet conditions in mind, it's true.
But, iron alloys that contain carbon are given to rust. Without getting technical, most alloys need carbon in them to form carbides which are the key to an alloy that will take and retain a good edge.
Recently, some new exotic alloys have appeared, Talonite comes to mind, which do take and retain a good edge while being virtually rust-proof. But, they are expensive.
The nice thing about the 100R&R is that while you certainly don't want to neglect it, it's inexpensive enough that if you lost it or something it wouldn't be a tragidy. So, there is that tradeoff.
Some alloys rust very quickly. I've heard of D2 rusting in a matter of hours. GIN-1 and 440C will not do this. You can go out and raft a river all day. Then, in the evening, just dry the thing off before you head to bed and it'll be fine. When you get home from your trip, clean the knife off, dry it off, and store it in the proverbial "clean dry place." DO NOT store it in the kydex sheath which could trap moisture around the blade.
The best way to protect a carbon steel knife blade from rust is to clean it, dry it, and then wax it. You can use ordinary car wax for this. But, while wax is durable enough for simple handling, wax is not durable for actual use. When you start using the knife, the wax and its protection will scrape or rub off.
If you have a knife that you want to throw into the trunk of your car or other place of emergencies, wax it. Then, when you need it, maybe years later, it'll be fine.
If you have a knife that you use infrequently, maybe a hunting knife that you use a couple of times a year, when you come back from that last hunting trip of the year, clean the knife up, dry it off, wax it, and put in away with the rest of your gear. Next year, when the season opens, your knife will be fine.
(Some folks may worry about using wax on a knife that they may be preparing food or dressing game with for fear of ending up injesting some of the wax. But, the amount of wax that will be on your knife is tiny, a very thin layer. Furthermore, the major ingredient in the sorts of wax products that I'm talking about that remains on the blade is carnuba wax. You can find carnuba wax as an ingredient in some food products (it's often a coating on gum balls and other similar candies to give a glossy shine). In other words, consuming a small amount of it is harmless and tasteless. If you really feel like you have to remove it, you can do so with Windex or other slightly ammonia window cleaner product.)
For a knife that you're going to use in the field, consider a product from Sentry Solutions called Tuff Cloth. This is a very good rust preventative. But, it also rubs off with use.
Finally, coating with a gentle oil as others have suggest here is a time-honored way to help prevent rust.
