The Royal Naval Division was a unique formation consisting of sailors serving on land as infantry.  Most of the men came from the Reserves, although there was also a Royal Marines Brigade. 

In 1916, the Division transferred from Admiralty to War Office Control and was renamed the 63rd (Royal Naval) Division (it also acquired an Army Brigade and various military elements).  However, Royal Naval Division records were treated quite separately and have their own files. These are now available, on microfiche, at The National Archives in Admiralty files ADM 339.

Generally speaking, the records of the men of the Royal Marines who served with the Royal Naval Division can be found in the holdings for the Royal Marines, whilst the records for the Army men can be found under the War Office.

The Fleet Air Arm Museum also have good holdings of enlistment papers relating to the Royal Naval Division.