The United States Navy

In 1834, the Marines came under the Department of the Navy. Historically, the United States Navy has had a unique relationship with the United States Marine Corps (USMC), partly because they both specialize in seaborne operations. Together the Navy and Marine Corps form the Department of the Navy and report to the Secretary of the Navy. However, the USMC is considered to be a distinct, separate service branch with its own uniformed service chief - the Commandant of the Marine Corps (CMC), a four star general.
The Navy and Marine Corps have a distinct relationship, the USMC depends on the Navy for religious support (i.e. chaplains) and medical support (i.e., dentists, medical doctors, medics known as "Corpsmen" and medical service staff); thus officers and enlisted sailors from the Navy fulfill these roles. When attached to Marine Corps units deployed to an operational environment they generally wear Marine camouflage uniforms, but otherwise they wear Navy dress uniforms unless they opt to conform to USMC grooming standards (appearance).
In the operational environment, as an expeditionary force specializing in amphibious operations, Marines often embark on Navy ships in order to conduct operations from beyond territorial waters. Marine units deploying as part of a Marine Air-Ground Task Force (MAGTF) operate under the command of the existing Marine chain of command. Although Marine units routinely operate from amphibious assault ships the relationship has evolved over the years much as the "CAG" (Commander of the Carrier Air Group/Wing) does not work for the carrier Commanding Officer (CO), but coordinates with the ship's CO and staff. Some Marine aviation squadrons, usually fixed-wing assigned to Carrier Air Wings (CVW) train and operate alongside Navy squadrons, flying similar missions and often flying sorties together under the cognizance of the "CAG". Aviation is where the Navy and Marine share the most common ground, since aircrews are guided in their use of aircraft by standard procedures outlined in series of publications known as NATOPS manuals.