I've found no evidence of USMC at McHenry. There is some easy confusion due to the existence of the "U.S. Sea Fencibles" which were odd units with the Commanding Officers reporting to the Army, presumably Gen George Armistead who commanded McHenry, but the troops were paid for by the Navy.
Pretty interesting article about them @
http://civilwarthosesurnames.blogspot.com/2009/01/united-states-corps-sea-fencibles-1813.html. There were two Companies of them there during the battle - one commanded by Capt. Matthew S. Bunbury and one commanded by Capt. William H. Addison
The Marines mostly fought in the Battle of Bladensburg where they, pretty much alone, performed well. Even then they were only 103 Marines, along with 300 sailors from the Chesapeake Flotilla commanded by Commodore Joshua Barney. There were only 1200 USMC at the time so nearly 10% of the force isn't small contribution.
