over one’s head vs overboard

over one’s head

overboard

verb
  • To throw over the edge of a boat into the water. 

adj
  • Outside of a boat; in the water. 

adv
  • Over the edge; especially, off or outside of a boat. 

  • Excessively; too much. 

How often have the words over one’s head and overboard occurred in a corpus of books? (source: Google Ngram Viewer )