"Du und ich schwimmen im Meer."
Übersetzung:You and I are swimming in the sea.
The continuous form describes an action that you are currently doing whereas the simple present is for general actions. Since it is impossible for you and me to swim in the sea forever/all the time, it's obvious for a German native that this action can only be temporarily done in this very moment. Thus you have to use the progressive/continuous form here.