There is a difference between a 96 hour rapid deployment force leaving CONUS. And a MEU prepostioned in a trouble area. The MEU is already underway. Has Organic fire Support and delivery assets, whether USN or USMC.
The MEU has reinforcing units ACB/AAB. Whose 1st echelons can be deployed in 24 hours. And Second & Third Echelons within 72 hours.
The Marine Corps has practiced this for years and has it down to a science.
As far as boarding vessels in time of war. Talk to some of the FAST guys or some of the FR guys they boarded vessels in the gulf in War I and are boarding them now.
That wasn't the only reason for the Marine Corps.
Secure and Maintain Advanced Naval Stations.
Maintain Order and discipline on Naval Vessels
Provide honor guards
Provide Security for Naval Yards
Provide crews to man the great guns
Provide Sentries to maintain discipline on the gun decks.
The Marine Corps provides an amphibous attack threat. The traditional duties of the Corps have continued to evolve past the traditional duties it held upon forming in 1775. But then so have the duties of the unit on which the Continental Marines were based, the Royal Marines.
Admiral Dupont said that a "Vessel without Marines is like a coat without Buttons."
Sgt. Rafael Peralta ,United States Marine Corps
Company A, 1st Bn, 3rd Marine Regt, 3rd Marine Divison
We will never forget your valor and sacrifice.
Semper Fi !