"The duck is swimming south."
Translation:Утка плывёт на юг.
"Плывёт" means "is swimming" while "плавает" indicates regularity or habit, so it would corresponds to something like "usually/typically/normally swims". So if the duck usually swims South (and then flies North) - your sentence can potentially make sense, otherwise - no.
плывёт is unidirectional - one-way, to a destination. In several months, the duck will swim, again one-way, north.
Except that ducks usually fly when they migrate, so that makes me wonder if the duck swimming "to the south" isn't just a particular direction right now, and the duck will swim in other directions as it navigates the lake or pond it is in. In that case, multi-directional swimming would be appropriate, I should think, and that would validate using плавает.
It should be reported. If a duck is swimming, it's just wandering around a body of water, it's not headed somewhere.