The hairy details will come later, I'm sure, but Russian verbs of motion (to go, to drive, to fly, etc.) have unidirectional and multidirectional forms. (It's the difference between "I am driving to New York" and "I am driving around New York". The verbs are different in Russian.)
Your first example, the verb is conjugated to mean "they are swimming (around)", while the second, it means "he is swimming (to a certain place)".
Also, you may wish to check if it actually says your second example, as that's a singular verb paired with a plural noun.