In earlier lessons yüzmek is translated as to walk, here and in my turkish grammar book it is to swim. Does it cover moving, and that the movement depends on context.
In earlier lessons (the somewhat similar) yürümek should have been "to walk." I used to get them mixed up a lot!
When completing the sentence "Ördek ...", is "Ördek yüzüyorlar" also a correct possibility? (meaning "Ducks are swimming")
Plural suffix can only be used for humans and not for other things. You can check the tips in the duolingo's desktop website :)