"Where are they swimming?"
Translation:Wo schwimmen sie?
A present continuous tense (like English “I am swimming”) doesn't exist in German. All actions in the present, current or habitual that they may be, are expressed with the present simple, so both “I swim” and “I'm swimming” should be translated with “ich schwimme”. “Sie sind schwimmen” is simply ungrammatical (something like “I am swim”). Context is generally enough to understand the meaning and when it isn't, adverbs (like “gerade”, “right now” or “meistens”, “usually”) can be used to disambiguate.