Structure of the United States Navy

The structure of the United States Navy consists of four main bodies: the Office of the Secretary of the Navy, the Office of the Chief of Naval Operations, the operating forces (described below), and the Shore Establishment.
Office of the Chief of Naval Operations.
The chief of naval operations presides over the Navy Staff, formally known as the Office of the Chief of Naval Operations (OPNAV). 
The Office of the Chief of Naval Operations is a statutory organization within the executive part of the Department of the Navy, and its purpose is to furnish professional assistance to the secretary of the Navy (SECNAV) and the CNO in carrying out their responsibilities.
The OPNAV organization consists of:
Policy documents emanating from the CNO are issued in the form of OPNAV Instructions.
OPNAV is one of the three headquarters staffs in Department of the Navy mainly based at the Pentagon, with the others being the Office of the Secretary of the Navy and Headquarters, Marine Corps.
Operating forces.
The operating forces consists of nine components:
Fleets in the United States Navy take on the role of force provider; they do not carry out military operations independently, rather they train and maintain naval units that will subsequently be provided to the naval forces component of each Unified Combatant Command. While not widely publicized, groups of ships departing U.S. waters for operational missions gain a Task force type designation, almost always with the Second or Third Fleets. On entry into another numbered fleet's area of responsibility, they are redesignated as a task group from that fleet. For example, a carrier task group departing the Eastern Seaboard for the Mediterranean might start out as Task Group 20.1; on crossing the mid-Atlantic boundary between Fleet Forces Command and United States Naval Forces Europe - Naval Forces Africa, it might become ('inchop') Task Group 60.1.
Numbered fleets.
The United States Navy currently has seven active numbered fleets. Various other fleets have existed, but are not currently active.
Additional numbered fleets have existed; for a period after World War II, the Sixteenth and Nineteenth Fleets were assigned as the reserve elements for Atlantic and Pacific Fleets.
Organization.
The organization of the Navy has changed incrementally over time. During World War II administrative organization for many ship types included divisions, for example Battleship Divisions (abbreviated BatDivs), Cruiser Divisions, Destroyer Divisions, or Escort Divisions (CortDivs, also rendered ComCortDiv for Commander, Escort Division), usually composed of two ships, often members of the same class. These made up squadrons (e.g. Battle Squadron, Cruiser Squadron, Escort Squadron (CortRon) etc.) of several divisions. Yet the exigencies of World War II forced the creation of the task force system where ships no longer fought solely as part of same-type divisions or squadrons. This was gradually reflected in administrative arrangements; by the 1970s, formations such as Cruiser-Destroyer Groups (CruDesGrus) came into existence.
The Navy is currently organized as such:
The Navy maintains several "Naval Forces Commands" which operate naval shore facilities and serve as liaison units to local ground forces of the Air Force and Army. Such commands are answerable to a Fleet Commander as the shore protector component of the afloat command. In times of war, Commander Naval Forces Korea becomes a Task Force (Task Force 78) of the United States Seventh Fleet. Other Naval Force Commands may similarly augment to become number fleet task forces.
The Shore Establishment.
The following shore-based bureaus, commands and components are directly subordinate to the Chief of Naval Operations:"
 
Relationships with other service branches.
United States Marine Corps.
Per 10 U.S. Code § 5001 (a) (2), § 5061 Introduction, § 5061 (4), and § 5063 (a): the United States Marine Corps, is (1) a separate branch of the naval service from the U.S. Navy; (2) the Department of the Navy and the U.S. Navy are distinct legal entities; (3) is, along with the U.S. Navy (and U.S. Coast Guard, when assigned) a component of the Department of the Navy; and (4) a branch of U.S. military service, separate from the U.S. Navy, within the Department of the Navy. Furthermore, per 10 U.S. Code § 5001 (a) (1), § 5061 (4), § 5062 (a): (1) the United States Navy does not include the United States Marine Corps (2); the U.S. Marine Corps is a separate component service, from either the U.S. Navy or the U.S. Coast Guard within the Department of the Navy; and (3) the U.S. Marine Corps is not a component of the U.S. Navy.
In 1834, the United States Marine Corps (USMC) came under the Department of the Navy. Historically, the United States Navy has enjoyed a unique relationship with the Marines, partly because they both specialize in seaborne operations. At the highest level of civilian organization, the USMC is part of the Department of the Navy and reports to the Secretary of the Navy. However, it is considered to be a distinct, separate service branch and not a subset of the Navy; the highest ranking Marine officer, the Commandant of the Marine Corps, does not report to a Navy officer. Marine Corps Medal of Honor recipients are awarded the Navy variant, and Marines are eligible to receive the Navy Cross. The United States Naval Academy trains Marine Corps commissioned officers while prospective Navy officers undergo instruction by Marine NCO Drill Instructors at OCS. Naval Aviation includes Navy and Marine aviators, flight officers, and aircrew.
The relationship extends to the operational theater as well. As amphibious assault specialists, Marines often deploy on, and attack from, Navy vessels; while being transported on Navy ships, they must obey the orders of the captain of the vessel. Marine aviation tailhook squadrons train and operate alongside Navy squadrons, flying similar missions and often flying sorties together. Other types of Marine air squadrons operate from amphibious assault ships in support of Marine amphibious operations. Navy and Marine squadrons use the same NATOPS aviation manuals and procedures. The USMC does not train chaplains, hospital corpsmen or medical doctors; thus officers and enlisted sailors from the Navy fulfill these roles. They generally wear Marine uniforms that are emblazoned with Navy insignia and markings to distinguish themselves from Marines. Corpsmen and chaplains enjoy a great sense of camaraderie with the Marines due in part because they work closely with them and often are embedded with Marine units. They operate under the command of the Marine Corps under the auspices of the Fleet Marine Force, often called the "green side".
Because of the lack of full-scale amphibious operations in recent conflicts, there has been pressure to cut the "gator navy" below the two-regiment requirement of the Marines. This is a reduction from the programmatic goal of 2.5 Marine Expeditionary Brigades and actual structure of 2.07 MEB equivalents in 1999.
The relationship between the US Navy and US Marine Corps is also one of mutual respect, and that respect is manifested in various policies and procedural regulations. For example, per US Marine and Navy drill manuals, in a formation consisting of both Marine and Navy units, per MCO P5060.20, Marine Corps Drill and Ceremonies Manual, Paragraph 15001. "ARRANGEMENT OF UNITS IN FORMATION 1. In ceremonies involving the U.S. Marine Corps and U.S. Navy units, the Marine unit shall be on the right of line or head of the column. The senior line officer, regardless of service, functions as the commander of troops." (As this is a Department of Defense/Department of the Navy regulation, no further 10 U.S. Code authority, other than already cited above, is required for the Secretary of the Navy, who supervises both the U.S Navy, and the U.S. Marine Corps, as well as the U.S. Coast Guard whenever it is assigned to the Department of the Navy, to specify that the Marine Corps takes precedence over the Navy and Coast Guard in Naval formations, parades, and ceremonies. This same military precedence is specified in DoD Instruction 1005.8 and U.S. Navy Regulations, Chapter 10, Paragraph 1007.) This is a symbol of the special status and honor granted to US Marines, and is a unique aspect of the Navy-Marine relationship.
United States Coast Guard.
Although the Posse Comitatus Act, which prevents federal military personnel from acting in a law enforcement capacity, applies only to the Army and Air Force, Department of Defense rules effectively require the Navy and Marine Corps to act as if Posse Comitatus did apply, preventing them from enforcing Federal law. The United States Coast Guard fulfills this law enforcement role in naval operations. It provides Law Enforcement Detachments (LEDETs) to Navy vessels, where they perform arrests and other law enforcement duties during Navy boarding and interdiction missions. In times of war, or when directed by the President, the Coast Guard operates as a service in the Navy and is subject to the orders of the Secretary of the Navy until it is transferred back to the Department of Homeland Security. At other times, Coast Guard Port Security Units are sent overseas to guard the security of ports and other assets. The Coast Guard also jointly staffs the Navy's Naval Coastal Warfare Groups and Squadrons (the latter of which were known as Harbor Defense Commands until late-2004), which oversee defense efforts in foreign littoral combat and inshore areas. Additionally, Coast Guard and Navy vessels sometimes operate together in search and rescue operations.