Marine units did serve in the European Theatre, but generally only as part of landing parties or as gunners on Navy ships. To my knowledge, no Marine unit served east of the Seine.
Barry said that his uncle served in the U.S. Army, as I recall.
There were Marines in the European Theater during WWII. As stated by Tony they served on ships as gunners and sharpshooters to take out enemy mines. They also guarded Naval Bases in Ireland. Small numbers of Marines were attached to the OSS and parachuted behind enemy lines to train the French Resistance fighters and saboteurs to harass the German war machine. They were not involved in liberating concentration camps.