Why a Marine division would not solve Normandy's landing problem

Would a US Marine division have captured the American Normandy beaches faster?

Not materially; the decisive constraints were defenses, lift, firepower, terrain, and logistics rather than the troops' lack of amphibious training.

The scenario adds an amphibiously trained Marine division without clearly supplying additional landing craft or specialist equipment. Army formations had already trained extensively, and commandos and Rangers were available for specialist tasks. The fortified beaches would still require the same difficult breach under the available fire support. Extra Marines are therefore expected to encounter much the same immediate constraints on the American beaches. A limited possibility of faster initial progress on different terrain is raised, but inland supply and joining up the landing forces would remain. Years of reconnaissance, including beach sampling by men brought in submarines, also undercut the idea that an extra trained formation could replace the preparation behind the operation.