I use an ultrasonic cleaner. Just make sure you use a strap stand to keep the watch above the surface of the water. Not sure of the effect an ultrasonic cleaner has on a submerged watch, and I don't wanna test it out on one of my watches.
For a genuine leather strap, coat the underside with neatsfoot or mink oil. Don't treat the topside, as the treatment will darken the leather. Once the oil penetrates, it protects against sweat, salt, and oil from your skin from drying and cracking the leather. You may also wish to treat the leather tops with leather conditioner. It will soften the leather and add years to the strap.
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