Believe it or not, knives can withstand water.
Most knives can in fact be washed with soap and water.
Some recomend the use of warm water as a way for the heated metal to hasten water
evaporation.
Unlike a fixed blade, folders tend to retain water by virtue of its sandwiched constructing.
The aim is to ensure that everything is a 100% dried out before storage.
Trapped moisture is a huge no, no.
No doubt that some materials which make up parts of a knife ( as in its handle or scales ), may be porous in nature and soak up water.
Thus its best to avoid knives with such materials if water damage is a cause of concern.
There are of course some after care products which may help to coat or seal a material's surface and repel water.
Prevention is better than cure