Translation:Many colorful ducks are swimming in the wide, deep river.
It is a bit annoying that only literal translations are accepted. In English somebody or something can legally swim "in" a river too. And don't tell me that the Hungarian is not "a folyóban", I think that here "in" is more appropriate than "on".
Ducks do swim ON the water and not IN. They're on the surface and not submerged. (apart from their little legs.)
Definitely in the river even though they are floating. „On the river“ suggests something geographically associated with the river, since this is an idiomatic use, as in place names such as Stoke-on-Trent or Stratford-upon-Avon. So in my humble opinion ducks would be common on but swim in the river.
It's of Slavic origin, but not specifically known to be from Ukrainian. A similar word exists in most Slavic language varieties (e.g. Polish kacza).