Diagnosis
Minor water ingress in the brush head coupling is common and usually not catastrophic. The handle is water-resistant, not waterproof at that joint.
How to Fix It
1. Immediately remove the brush head.
2. Shake the handle vigorously to eject water from the coupling.
3. Stand the handle upright (button on top) on a towel in a well-ventilated area for 24-48 hours to dry internally.
4. Do not use a charger or attempt to turn it on during this drying period.
If That Didn't Work
If after drying the brush exhibits erratic behavior, fails to hold a charge, or shows moisture under the button seal, water has reached the electronics. Damage is likely permanent.
Heads Up
Prevent this by rinsing the brush head separately before attaching it, and by avoiding submerging the handle. Water in the coupling can also lead to mineral buildup and a stuck brush head.