Still seems unnecessary to me, what is the point of raiding or even seizing a couple of Adriatic islands in 1944? The constraint on SEAC amphibious operations was not manpower, it was amphibious shipping, in fact this was generally true for all British theatres. Better to use the manpower to build & crew extra landing craft and shipping more generally rather than have a large raiding force that can't actually do any raiding for lack of shipping. While on the subject of manpower, just like the army and the navy many marines were employed in what were essential jobs that didn't involve being part of a ship's crew or fighting force, if the RM personnel were relieved of these duties then they would have to be replaced somehow.
Look up the use of Vis/Lissa in 1943-45; the RMD in the Adriatic might have actually been just the tool to help eliminate the coastal routes for Army Group F as it withdrew from the Balkans. In 1944, the RMD could have been assigned to BRASSARD, SITKA, or ROMEO, or it could have been very useful in the Aegean as the Germans withdrew in the autumn of 1944; cutting off (for example) the German 22nd Division on Crete, or the German garrisons in the Dodecanese, as late as 1945. Given that the RN was able to commit a strong enough escort carrier force to the Aegean in 1944 that the German advantage in land-based air power was negated (unlike in 1943), there are multiple possibilities of taking effective action.
A SEAC deployment opens the door to an "early" (1944) assault on Akyab and/or Ramree; a SoWesPac deployment gives the Commonwealth an amphibious force worth the name prior to OBOE in 1945; presumably there are multiple locations where such a force could be useful.
Again, as far as candidates from landing craft crews in 1943, one would think that
Revenge and her sisters, much less the
Ceres,
Danae, etc. class cruisers, would be more obvious pools of trained sailors than an infantry division. Same for the
Banffs and the remaining RN-manned "Town" class destroyers.
Given the British Army's manpower crisis in Europe in 1944-45, simply using the RMD as
infantry, rather than small boat crews, seems like an obvious path, for that matter. 101 or 102 RM brigades could have been beefed up with shares of the RMD's divisional assets, and used to augment or replace army infantry brigades; as it was, the British broke up three divisions to yield an equivalent of seven infantry brigades of replacements, and added the previously independent 231st Brigade to the 50th Division in Sicily and the 56th Brigade to the 49th Division in Normandy. Presumably folding the two RM brigades into 15th or 21st army groups can keep either the 50th or 59th infantry divisions, or the 1st Armoured Division, functional as such.
If the RMD goes out to the east, and the 29th Brigade is attached, it could replace the extemporized 36th Division.