understate vs water something down

understate

verb
  • To state (something) with a lack of emphasis, in order to express irony. 

  • To state (something) with less completeness than needed; to minimise or downplay. 

  • To state a quantity that is too low. 

water something down

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