It depends if you want to speak in generalities or of a specific time. How are the dogs spending their summer afternoons? "They are swimming" nadan What are the dogs doing right now? "They are swimming" estan nadando
Why I need to write an article if IN GENERAL all dogs are able to swim? If you see a group of dogs in a specific situation is not more common to use "the dogs are swimming? (I'm Learning spanish from english base, my L1 is Italian) Thanks you for your attention.