Leather harnesses are unfit for swimming, bathing and outings in heavy rain. If the harness is drip with wet, use a dry cloth to wipe it and let dry at room temperature. Oil the harness when it is dried with a balm.
It is better to keep the harness in a special bag of natural material, which is ventilated. This will save the item from excessive humidity, dryness and dust.
Use leather care products from time to time to keep the harness flexible, soft and shiny.
Read how to care for leather harness, muzzle, collar and leash?