FAQ: Should I protect my canvas and leather from water?
You don't need to be paranoid about getting caught in every little rainstorm but, unless your item is specifically made out of a water-resistant material like waxed or rubberized canvas, oiled leather, or naturally water-resistant skins like goatskin or pigskin, you'll want to avoid letting it get soaked through. The water will weaken the fibers, making the item less supple and prone to tearing. Salt water is corrosive and can be particularly damaging.
If your travel gear does soak through, the solution is to take action, cleaning and conditioning it before it completely dries. Also don't try to rush the drying process - just spread it out and let it air-dry at its own pace.