OK, Got to level 6 and eventually got the crane swung around, stepped on the footplate between the totems to reveal an underwater door, but can't seem to get it open - any suggestions ?
  
 
  
  
    swim, swim, SWIM.... well, till you see the
 underwater airplane. Snatch the propeller
 (you'll need to bang it with the hammer) and
 use that to pry open the underwater door.
 
 bon