I recently got an Apple Watch, and the other day I noticed the crown dial (aka the "little spinny knob thingy") was suddenly hard to rotate, and not very responsive to clicks.
After a little thinking and experimenting, I remember that I'd been using the Watch to record workouts and it's been particularly hot in Portland lately. It seems that the minerals in my sweat were accumulating under the crown knob and making interfering with its proper operation.
Wash away your troubles
If this happens to you, the fix is easy. Simply rinse off the watch in warm (not hot) water with mild soap. Rinse the watch well and, while rinsing it, rotate the knob a bit to get the mineral crystals worked out of the mechanism.
I've done this a couple of times now, and it fixes the problem easily - in fact, since I have a sport band (the rubbery one) I think I'll just wear it in the shower to keep it clean. The watch is water-resistant and this kind of exposure should not be a problem. According to Apple:
Apple Watch is splash and water resistant but not waterproof. You can, for example, wear and use Apple Watch during exercise, in the rain, and while washing your hands, but submerging Apple Watch is not recommended. Apple Watch has a water resistance rating of IPX7 under IEC standard 60529. The leather bands are not water resistant.
By the way, there are also articles from other sources discussing tests that indicate that the Apple Watch is even more water-resistant than Apple's claims indicate.