Assuming you get as far as Recruit training, when you get issued your nice new shiny ID cards, I think you'll find, in nice big letters (for the hard of hearing) the words "ROYAL NAVY" across the top. As such they are part of the Senior Service & all that entails. The Royal Navy is extremely proud of the Royal Marines, & justifiably so.
The RM, although a self sufficient Corps, usually operate with the Navy providing their means of delivery, the Army their means of support, both logistics and close-air, together with top cover from the RAF.
All three arms of the Armed Forces usually act together, so their pedigree is purely academic. For a long time the Army have sought "ownership" of the Corps, so it is very doubtful they would wish to become a fourth Junior Service, subservient to the other arms in my opinion.
It's rather worth studying the Corps History and that of Sea Soldiers from the British Army, all will become clear.
