Spielberg got lucky the mechanical shark broke, which turned the B-movie slasher into an all-time classic. Robert Shaw (Quint) was totally alcoholic IRL, and turned in one of the great monologues of the 1970's drunk as a skunk.
This is similar to George Lucas having total SH*T for principle photography for Star Wars, and necessitated the greatest editing job in Hollywood history for which his wife (at the time) won an Oscar.