What are those arrowlike underwater structures for?
Those are Yap Traditional Fish Weirs, according to islandculture.info


  In 2008, Yap State Historic Preservation Office (YSHPO) commenced a
  project to survey the fish weirs (aech) around the main island of Yap.
  The aech is a stone/rock structure, usually in the shape of an arrow
  (many have a shaft, some do not), that catch fish after they move
  inside a catchment area at high tide and are trapped when the tide
  goes out. It is thought 700-800 aech may be located around Yap's main
  island which has a coastline of about 100 km in circumference.