What exactly is the mission of the US Navy? Well, I will tell you.

The mission of the Navy is to maintain, train and equip combat-ready Naval forces capable of winning wars, deterring aggression and maintaining freedom of the seas.
—Mission statement of the United States Navy
From the New Recruits Handbook:
The mission of the United States Navy is to protect and defend the right of the United States and our allies to move freely on the oceans and to protect our country against her enemies.
The United States Navy serves as a seaborne branch of the Military of the United States. 10 U.S.C. § 5062 prescribes the navy's three primary areas of responsibility:
• "The preparation of naval forces necessary for the effective prosecution of war"
• "The maintenance of naval aviation, including land-based naval aviation, air transport essential for naval operations and all air weapons and air techniques involved in the operations and activities of the Navy"
• "The development of aircraft, weapons, tactics, technique, organization, and equipment of naval combat and service elements".
U.S. Navy training manuals state the mission of the U.S armed forces is "to prepare and conduct prompt and sustained combat operations in support of the national interest". As part of that establishment, the U.S. Navy's functions comprise sea control, power projection and nuclear deterrence, in addition to "sealift" duties.

The above comes from the US Navy Official site and Wikipedia. All naval personnel must complete training on the Navies mission for advancement. As the service members rank increases the knowledge of the mission also becomes more involved and intense.