Why isn't it "Bent jij aan het zwemmen?" seeing as you are referring to a second person?
If the verb is in front of je/jij it takes the verb belonging to ik. So: Jij loopt naar huis - Loop jij naar huis?
In English, there are three reasons to ask "Are you swimming?" 1) Are you walking across the lake, or are you swimming? 2) When holding up and encouraging a toddler to learn the skill 3) When the answer is 'Stupid question. No, you fool, I'm drowning. HELP!". I am pretty sure the intent of thisquestion in Dutch is better expressed as "Are you GOING Swimming?".