Water itself does not usually dull a knife overnight, but the habits around washing and storage can do real damage. Letting a knife sit in the sink invites accidental knocks against other utensils. Tossing it loosely into a drawer means the edge repeatedly rubs against metal, ceramic, or even other blades. Over time, that contact chips and blunts the edge.
Dishwashers can be even harsher. High heat, strong detergent, movement against other items, and long exposure to moisture can all shorten a knife’s life. Add to that the risk of the handle loosening or the blade corroding, and it becomes easy to see why hand-washing is usually the safer choice.
The better routine is plain and boring, which is often the best kind of routine: wash by hand, dry immediately, and store the knife in a block, magnetic strip, or sheath. A blade that is protected between uses will almost always outlast one that is left to rattle around with spoons and forks.
