A duck spends a lot of time preening, combing, spreading oil over its back during its downtime. Have you noticed?
The water does slide so easily off their backs when they surface after diving down. It is admirable how easily they shed it. Droplets fly off, nothing sticks. The ducks can go about their business.
But that is because they spend a lot of time caring for their backs, calming/combing the feathers down. They know water will be there. They take the time to prepare for it.