Simple question: How autonomous is the Marines from the Navy?
They can’t get very far from A to B without the Navy. Also, Marines do not have medics - they use Navy corpsmen.
Isn’t there much more to it administratively then that? I’m just thinking of a Few Good Men where Cruise, a Naval JAG officer, is trying two Marines. I know I shouldn’t get my information form Hollywood, that’s why I ask.
than that . . . from Hollywood
Well, the Marines are rather dependant on the Navy - after all, MARINE is stands for "My A** Rides In Navy Equipment. " 
The US Department of Defense is split into three Departments: Department of the Air Force, Army, and Navy. As the military head of each department, you have a Chief of Staff (these Chiefs serve as the Joint Chiefs of Staff). Each of these Chiefs of Staff reports to a civilian – the service secretaries.
The Navy’s Chief is called the Chief of Naval Operations, or CNO, and is a 4-star admiral who reports to the Secretary of the Navy. The Commandant of the Marine Corps is also a 4-star (general). The Commandant of the Marine Corps sits on the Joint Chiefs, and reports to the Secretary of the Navy just as the CNO does.
So, at least administratively, they are co-equal services within the Department of the Navy, and have the same level of autonomy as any other service. Functionally, they are just as able to function without the Navy as the Army is able to function without the Air Force – not very.
As Jurph says, they are part of the Department of the Navy. Their ID cards (well, at least the old ones) had a nice big “Department of the Navy” across them. Of course we Sailors liked to rib them on this.
It’s not that odd, historically. The Air Force branched from the US Army Air Force, for example. Truckloads of information about this stuff online.
As a Navy vet we used to joke about shelling the beach for a while then going to drop off a few hundred Marines.
We would go off the coast a bit sit in the AC, take hot showers and play cards until it was time to go pick up whoever is left.
They can call us a taxi service all they want and I’m fine with it.
Logistically the Marines are a short term expaditionary force. They go in and blow up shit and kill people like there’s no tomorrow. Without logistical support from the other branches they can’t stay for long.
The current Chairman of the Joint Chiefs of Staff is a Marine - General Peter Pace. He’s been in that post for less than a month - link.
He is the first Marine to hold that job, and I seem to recall that it was regarded as a milestone in some circles - it removed one of the remaining arguments/perceptions that the Marine Corps is “under” the Navy.
Others are probably more suited to comment on the significance of this than I am.
Gangster, in direct answer to your question, they are very autonomous. Day to day, both in the field and in the Pentagon, you’d never know that they were technically part of the Navy. They are for all intents and purposes, their own service.
They’re just very, very good friends.
“Anytime we gotta go somewhere, you guys give us a ride!” <insincere fuck-you grin>
- Lt. Kendrick, A Few Good Men
…who rag on each other constantly and get in your occasional in-house barfight, but will stand behind one another against any outsider. More like brothers, actually.
If you ask a Marine, “Amused contempt”.
Also, the Navy and Marines share a service academy, go through the same flight schools (through type selection, and beyond in some cases), etc. Opinions vary depending on what part of each service is considered, but I’d guess that many sailors and marines, while they might bicker between themselves, would align against either air force or army.
One thing that I always found interesting, given the common misconception that the Marines are a subset of the Navy, is parade precedence:
Basically it boils down to traditions surrounding the order of instantiation of each service.
I have to say that the Navy is a little older than the Marines.
from here a Navy faq page.
from here the USMC homepage.
It’s not a misconception. It’s a fact.
I thought Marines were the bastard stepchildren of the Navy? Or is that only the Royal Marines 
It really is a misconception. Like Jurph said, The U.S. Marine Corps and the U.S. Navy are coequal branches of the military under the U.S. Department of the Navy. The Commandant of the Marine Corps isn’t under and doesn’t report to the Chief of Naval Operations, they both report to the Under Secretery and the Secretary of the Navy. Check out this chart:
Blah. The Navy and the Marines are part of the Department of the Navy. Let’s not pretend here that Navy here is used in two different senses. It means the same thing in both cases. Fact of the matter is that both the CNO and the Commandant report to the Secretary of the Navy. The fact that the CNO and Commandant are equal is irrelevant. There is a near-1-1 matching in ranks between all of our services, and many foreign services as well. It doesn’t change the fact that historically the Marines were created as landing troops for the Continental Navy and that today they pretty much serve the same role.
I really don’t get this argument. “But, the Marines and the Navy are part of the Department of the Navy, ergo, the Marines aren’t part of the Navy!” huh?