And because machine guns were deemed, doctrinally speaking as it were, to be an 'artillery' asset, whereas the very purpose of the Royal Marines (RMLI fm 1855) was to function as infantrymen. I am not clear on the size and distribution of the RMA (founded 1859), but they were not as I understand it to be found in that many ships' complements, in the same way that RMLI parties were. Where several ships contributed to a larger naval brigade ashore, the RMLI contingents seem usually to have been grouped together to function conventionally as companies of infantry or, in a big show, as a battalion of as many companies as the aggregated manpower would permit. Where the RMLI was not present in sufficient strength to function as described, they typically fulfilled the skirmisher function in support of musket/rifle armed bodies of ratings. A good example of the latter method is Peel's naval brigade ('the Shannons') in the Lucknow campaign. Interestingly, but of no real relevance to your question, a couple of hundred of Peel's men were not actually in the Royal Navy at all, but were merchant seamen from the ships anchored at Calcutta who volunteered to serve ashore with the brigade. If memory serves they started upcountry a couple of weeks after Peel had departed with the professionals, during which period they were given some rudimentary infantry training under Peel's first lieutenant and a couple of seconded infantry NCOs. At the front the sailors manned the 24-pdrs and operated as infantry companies, while the 50 odd RMLI did the skirmishing thing in front. Thus when machine-guns came along a couple of decades later it was natural that they be manned by ratings alongside the artillery pieces (doctrinally not tactically 'alongside' that is). Of course the RN was rather more keen on machine guns than the Army was. Wolseley felt that a machine gun (in the pre-Maxim age of course) could not do anything that a section (20-25) of riflemen could not. Hiram Maxim would change all that, and the Army's attitude with it.
Dr Mike Snook MBE psc