add water to vs water down

add water to

water down

verb
  • To simplify or oversimplify; to make easier; to make less difficult. 

  • To make weaker, less effective. 

  • To dilute; to add water to. 

  • To make less restrictive; to make more lenient. 

How often have the words add water to and water down occurred in a corpus of books? (source: Google Ngram Viewer )