To my mind captions "oni pływają", "oni chodzą" and "dzieci biegają" have two meanings. First one is that they are doing this activity at this particular time, it's like: they are swimming, they are walking (they are doing it right now) Those captions can also mean that they swim, they walk, so it's like in present simple(this is the activity they usually do or they are able to do).
"Oni płyną", "dziewczynki idą" and "kobiety biegną" have only one meaning: they are swimming, girls are walking, women are running. Here we don't have this second option;
I hope it'll help you a little ;-)