Restaurants in highly touristy areas are interested in giving you a tasty meal AND in turning over the tables. Pick a nice outdoor taverna, enjoy the ambiance. You will get served quickly and be on your way.
Why don't you visit Avignon or Les Baux or Arles?
You have already been to Aix and Cassis is just a little town without the star power of those others.
Could you change the timing of the catamaran to a later time? In that way you could take an early ship's tour starting at the ferry port, take a taxi to catamaran port (Vlyhada?), and get dropped off at the tender port at the end. I know there are lots of ifs and wishful thinking, but maybe you can make it work.