The phrase 'Rock the Boat' means to do something that causes trouble or disturbs the harmony of a situation.
Example of Use: "We are days before the elections; you can't rock the boat now!"
The origin of the idiom 'rock the boat' is attributed to American statesman, lawyer and politician William Jennings Bryan, who was famously quoted “The man who rocks the boat ought to be stoned when he gets back on shore.” In this 1914 quote, it is evident that Bryan was referring to those who stir up trouble.