Unable to locate water resistant mode on Apple Watch

Press the side button (not the crown) to open the Control Center and tap the "water drop" icon. If you don't see it, tap Edit (at the bottom of the screen) to add it to the Control Center.

The Apple Watch is ALWAYS water resistant.

The water mode disables screen & button touches so you do not accidentally call or text someone, aka “But Dialing”.

How to use Water Lock and eject water from your Apple Watch - Apple Support

Just to confirm the advice from BobHarris, the Water Lock function simply makes the screen ignore touches and water splashes. Water Lock should usually detect and activate automatically if you start swimming with the watch. An electronic function can never change the gaskets and seals the watch uses to prevent water getting into the electronics.

Because manual operation is now rarely needed Apple has removed it from the default buttons in Control Center to reduce clutter. As noted above, you can add it back if you wish.