Do you supplement niacin? Mixing the duckling feed with the chick feed is going to reduce the niacin percentage a little.
If she's getting a lot of treats, that's also going to water down the nutrients from the feed a bit
She's still young. Is she preening herself when she gets out of the water? Does her oil gland looking okay?
Ducklings shouldn't be unsupervised in water until 8 weeks.
It definitely isn't safe for her to be out free ranging on a river. Predators, drowning, it's all a possibility.