Complex sentence: Besides the Royal Navy, the Royal Fleet Auxiliary and the Royal Marines operate their own fleets of naval vessels which contribute to the available seagoing assets of the Royal Navy, however they are not included in this list or the above figures.
Split into simpler sentences: Besides the Royal Navy, the Naval Service also includes the Royal Fleet Auxiliary and the Royal Marines. Both of these constituent parts of the Naval Service operate their own fleets of naval vessels which contribute to the available seagoing assets of the Royal Navy, however they are not included in this list or the above figures.