Historically, the Royal Marines were, to be blunt, used questionably by the British high command in WW 2. In a war that was defined, in large part, by amphibious warfare, the RM spent most of the war years in their traditional shipboard roles of gunners/etc. (a capital ship generally had 100 to 200 RMs aboard), security/garrison needs in the UK and other established naval bases, or their interwar-developed role of a combined arms defense force for overseas naval bases (the "Mobile Naval Base Defense Organization," or MNBDO), which were - essentially - brigade-sized mixes of infantry, coast artillery, AA artillery, etc. who were "mobile" the sense they could be deployed to build-up a functioning naval base in an otherwise austere location, but were "static" once they arrived.
In WW 2, the RM increased from some 13,000 officers and men (regulars and reserves) in 1938 to 82,000 in 1943, and along with providing detachments for the larger ships of the RN, security and garrison duties, and the manpower sufficient for two MNBDOs, designated (creatively) MNBDO I (established prewar, and designated as such in 1939) and MNBDO II (established in 1941). Through a complex series of deployments and detachments, much of MNBDO I ended up serving in the Indian Ocean and much of MNBDO II in the Med, although elements of I also served in the Med and elements of II also served in northwest Europe. These two formations gave rise to the 1st, 2nd, and 5th RM AA brigades, of which the 5th was (basically) a conglomeration of the 1st and 2nd for service in NW Europe in 1944.
By the end of 1943, RM manpower broke (roughly) down as follows:
14,000 - landing craft crew;
11,000 - headquarters and depots;
8,000 - Fleet duties (shipboard detachments);
7,000 - Special service (commandos);
42,000 - RN base duties, FAA, training units, RM engineers, Allied/joint/combined staffs, etc.)
It is worth noting that in a conflict defined by amphibious assaults, only about 7,000 RM were assigned to what amounted to light infantry units trained and equipped for amphibious assaults...
It is also worth noting that the majority of those 7,000 RMs were late arrivals to the "commando" role; before 1943, only two commandos (basically, an understrength infantry battalion) had been made up of RM personnel; most of the commandos were made up of soldiers before 1943. The first RM unit was originally formed as "The RM Commando" in February, 1942, and then redesignated as 40 RM Commando; the second was formed in October, 1942, as 41 RM Commando, largely by redesignating the existing 8th RM Battalion (infantry).
In mid-1943, the majority of the RM's land warfare capabilities were found in an understrength light infantry division designated as "The Royal Marine Division," formed as such in August, 1940. On paper, its organization was (more or less) as follows:
RM Division headquarters - CG: MG Sir Robert Grice Sturges
Division Troops: 15 RM Battalion (MG); 18 RM Battalion (Mobile); RM Division Engineers; RM Division Signals; etc.
Formation headquarters
101 RM Brigade (hq); 102 RM Brigade (hq); RM Division Artillery
Infantry:
1st RM Battalion; 2nd RM Battalion; 3rd RM Battalion; 5th RM Battalion; 10th RM Battalion;
Artillery:
RM Field Artillery Regiment
RM Anti-tank Regiment
Amphibious Engineers:
RM 1st Beach Group
RM 2nd Beach Group
Two more RM infantry battalions existed; the 7th was in the Med as an independent battalion, and had absorbed the 9th RM Battalion in May, 1942, while the 11th RM Battalion was in the Indian Ocean.
Now, historically, the division sketched above was converted in 1943 to either a) commando units (the historical 42-48 RM Commandos), or b) landing craft crews (the 14,000 men mentioned above). The division headquarters was converted from an operational hq to an administrative one, becoming headquarters of the Special Service Group in the UK in August, 1943, while the brigade headquarters provided personnel for elements of what became the four Special Service/Commando brigades of 1944-45. In 1945, two more brigade headquarters - 116 and 117 RM Brigades - were formed, using a mixed group of personnel, to provide additional light infantry for NW Europe.
There were a fair number of reasons - doctrine, the British conscript manpower crisis of 1944-45, inter-service rivalries, etc. - as to why all of the above occurred, but it certainly raises the issue of whether the mass conversion of seven RM infantry battalions to seven RM commando battalions, and the provision of 14,000 landing craft crew, was really the best use of trained light infantry like the RMs.
So, here's the POD: in the summer of 1943, rather than the conversions and re-assignments outlined above, WSC, Bourne and Hunton (the the senior RM generals), the RN, and the Army agree the Navy can find landing craft crew from the fleet, and the two extant RM battalions and the Army's commando battalions (1-4th, 6th, and 12th in the UK, 9th in the Med, and 5th in the Indian Ocean, as well as various Allied, "local," or special units) can handle the commando role as is necessary.
For example, decommissioning the four remaining Revenge class battleships would yield 4,000 RN officers and men, and the 14 remaining WW I-era light cruisers (two Emerald, two Frobisher, six Danae class, four Caledon+ cruisers) another 7,000, roughly; the last 3,000 presumably could have been made up by transferring the seven remaining Banff-class sloops back to the USCG (1400 men total), as well as many of the remaining ex-USN "four piper" destroyers the British chose to decommission (~150 men each), so 11 of the 41 that historically survived until the end of the war.
Those post-POD decisions yield a light infantry division drawing from a pool of (at least) 19,000 men (14,000 landing craft crews plus at least 5,000 light infantry NOT converted to commandos), organized (presumably) as follows:
RM Division headquarters
Division Troops: 15 RM Battalion (MG); 18 RM Battalion (Mobile); RM Division Engineers; RM Division Signals; etc.
Formation headquarters
101 RM Brigade (hq); 102 RM Brigade (hq); RM Division Artillery
Infantry:
1st RM Battalion; 2nd RM Battalion; 3rd RM Battalion; 5th RM Battalion; 10th RM Battalion; add 7th or 11th as appropriate, given a decision on deployment)
Artillery:
RM Field Artillery Regiment
RM Anti-tank Regiment
Amphibious Engineers:
RM 1st Beach Group
RM 2nd Beach Group
That's roughly two brigade groups, with 7-8 maneuver battalions between them, and artillery and engineers to match. Seems like a useful formation for the Med or Indian Ocean, in 1943 and afterwards...
So, if the above all occurs, and both the point to point and the amphibious assault shipping is available in 1943-44, where do you send it?
Scotland or Iceland, to provide something tangible to back up FORTITUDE NORTH? The Med, to provide a British force sized for BRASSARD or something similar? The Indian Ocean, to give Mountbatten an amphibious division - presumably beefed up with the Army's 29th Independent Brigade) a year early?
Or how about all the way to the Southwest Pacific, to reinforce the theater in anticipation of the withdrawal of the (US) 1st and 3rd Marine divisions for the Central Pacific in 1944?
Thoughts?
Sources:
The Royal Marines and the War at Sea, 1939-45, by Watts, Martin
and
The Watery Maze: The Story of Combined Ops, by Fergusson, Bernard