Why Royal Marines served aboard warships

Were Royal Marines carried aboard warships mainly to prevent mutiny?

No; their main shipboard purposes were to man guns and supply trained landing parties, while the accommodation layout weakens the mutiny-police explanation.

The suggestion that Royal Marines were stationed aboard chiefly to suppress mutiny does not fit the shipboard arrangement described here. The locations of Marines, officers, crews, and secured weapons argue against that simplified explanation. Their principal duties included manning part of the ship's armament and providing organized landing parties. Those operational jobs explain why Marines were valuable to a warship without relying on the later mutiny-police story.