When playing the road building card, can a settlement be built between a free road and a free ship?
No, it cannot. From the official Catan FAQ:


  Development Cards – Progress – Road Building - May I use a Road Building card to build 1 road and 1 ship?
  
  
    Yes. You must build them one after another though. You are not allowed to first build a road via a Road Building card, then build a settlement adjacent to that road by paying the required resources, and finally use the Road Building card once again, this time to build a ship.