The US Navy has its own ground combat force. No, not the Marines:
The marines are not part of the navy, as they are often described. Both the navy and marines are part of the Department of the Navy. The Department of the Army and Department of the Air Force each have only one component while the Navy Department has two (the fleet and the marines) who are separate services that are closely intertwined. For example, the navy provides many support functions for the marines which, in the army and air force, are provided by each service. Thus navy personnel serve in marine units (wearing marine combat uniforms) as medics and other support specialists. In the army the medics are soldiers and the air force support personnel are all airmen. The use of the navy for support functions means a much higher proportion of marines are combat troops than in the navy, army or air force. This gives the marines a different attitude and outlook.
Over the years, the marines have acquired more and more autonomy from the navy. When the U.S. Marine Corps was created, over two centuries ago, marines were sailors trained and equipped to fight as infantry, and they were very much part of the navy, and part of ship crews. This changed radically in the late 19th century, when all-metal steam ships replaced wooden sailing ships. The new “iron ships” really didn’t need marines, and there were proposals to eliminate them. In response, the American marines got organized and made themselves useful in other ways. For example, the marines performed very well as “State Department Troops” in Latin America for half a century (late 19th century to just before World War II), where American troops were frequently used to deal with civil disorder abroad and nation building. During World War I (1914-18), they provided a brigade for ground combat in Europe where they demonstrated exceptional combat skills.
As World War II approached during the 1930s the U.S. Marine Corps really ran with the ball when the navy realized they would have to use amphibious assaults to take heavily fortified Japanese islands in any future war. Thus, once the U.S. entered World War II, the marines formed their first division size units, and ended the war with six divisions, organized into two corps.
The Marine Corps was no longer just a minor part of the navy, but on its way to being a fourth service. Over the next half century it basically achieved that goal. But in doing that, the navy lost control of its ground troops. Navy amphibious ships still went to sea with battalions of marines on board. But because the marines are mainly an infantry force, and the war on terror is basically an infantry scale battle, the marines spent a lot more time on land working alongside the U.S. Army.
In response to all this U.S. Navy began building a new ground combat force in 2006, staffed by 40,000 sailors. This is NECC (Navy Expeditionary Combat Command), which is capable of operating along the coast and up rivers, as well as further inland. NECC units have served in Iraq, and are ready to deploy anywhere else they are needed. The 1,200 sailors in the EOD (Explosive Ordnance Disposal) teams are particularly sought after, because of increased use of roadside bombs and booby traps by the enemy. NECC organized three Riverine Squadrons which served in Iraq. NECC basically consists of most of the combat support units the navy has traditionally put ashore, plus some coastal and river patrol units that have usually only been organized in wartime.