Cleaning a watch is safe as long as you respect two things: whether it's actually water resistant, and whether the crown is pushed in. Get those right and a quick wipe-down won't hurt anything. Get them wrong and you can force moisture past a seal that was never built to handle it. Here's how to do it without turning a five-minute chore into a repair bill.
Check the water resistance rating before you touch water
A watch marked 3ATM or "water resistant" without a number is not meant for anything beyond incidental splashes. For those, skip the water entirely and clean with a barely damp microfiber cloth, then dry it right away. If your watch is rated 5ATM or higher, you can rinse it under a light stream of lukewarm water and even use a drop of mild soap on the bracelet and case. Never use hot water, since heat expands gaskets and seals in ways that shorten their life.
Vintage pieces are the exception no matter what the case back says. Gaskets degrade over decades, and a vintage Seiko 5 that was rated for swimming in 1975 shouldn't be trusted with more than a damp cloth today unless it's been serviced recently. When in doubt, treat older watches as splash-resistant only.
Never clean around an unscrewed crown
The crown is the weak point in any water resistance system. If you've pulled it out to set the time or date, push it back in and screw it down, if it has a screw-down crown, before any cleaning begins. Water and soap that get into the movement through an open crown tube cause far more damage than anything sitting on the case exterior ever could.
For the bracelet or strap, a soft toothbrush works well for getting grime out of links or between lugs. Leather straps should never get wet. Wipe them with a dry cloth only, and let sweat and oils be the enemy you manage with rotation, not water. Dry everything with a soft cloth immediately after cleaning, since trapped moisture is what actually causes corrosion, not the water itself.