The Question and Answer section for Johnny Tremain is a great resource to ask questions, find answers, and discuss the novel.
This issue unites the thirteen colonies against the British, The British ruled by dividing and conquering: pitting colony against colony. The Boston Tea Party galvanized all the colonies against a common enemy.
The Sons of Liberty manipulate public opinion by holding demonstrations, marking their intent that force and violence would be avoided unless absolutely necessary. They also control the newspaper.
All these ships carry tea. The rebels plan to throw the tea overboard hence the famous "Boston Tea Party".