The Marines are one of the core organizations in the One Piece world and can be considered as the navy of the World Government, tasked with the responsibility of policing the seas and conducting war operations. As one of the main antagonist groups in this series, they have an important place in the development of the plot and challenging the main protagonists.
The Marine organization is structured with a well-defined hierarchy of ranks, where specific duties are attached to every rank with their levels of authority. At the top sits a Fleet Admiral who holds the highest rank and leads the entire organization. Currently, Sakazuki, popularly known as Akainu, holds the position - an infamous and dreaded justice hunter.
Immediately below the Fleet Admiral, the Admirals number three in most cases. Normally, they are among the most powerful characters in the One Piece universe, with proficient martial skills and frequently user of devastating Devil Fruit powers. These include famous Admirals such as Kizaru, Fujitora, and Ryokugyu, all able to bring their different skill sets and philosophies to the position.
The Vice-Admiral position is the third highest rank in the entire Marines. Moreover, a Vice Admiral himself has different places with different levels of powers in the institution. This looseness in roles is what allows a great number of characters and plot diversity regarding Vice-Admirals throughout the series.
Below these top ranks come such positions as Rear Admiral, Commodore, Captain, Commander, Lieutenant Commander, Lieutenant, and Lieutenant Junior Grade. All of these ranks fulfill some sort of duty that keeps the Marine organization running, from commanding ships to leading troops on the ground.
It is also important to note that the rank of a Marine officer commanding a ship is not necessarily "Captain," contrary to most naval conventions. This fact definitely makes the structure of the Marines complex and a far-from-real naval organization.
The main weapons at the disposal of the Marines are firearms and melee weapons. Rifles are standard-issue to most of the Marine infantrymen and favored by snipers. Most of the guns shown in One Piece are Flintlock muskets and pistols, resembling the guns of the 18th and 19th centuries. Still, such kinds of weapons were capable of doing more than their authentic ancestors; for instance, they could shoot without reloading. All this brings a very cool look into One Piece and a feel to the whole narrative.
The relationship between the Marines and other factions in the One Piece world is pretty complicated and strained at times. To illustrate, many high-ranking Marines scorn the Seven Warlords of the Sea, a group of state-sanctioned pirates, and view them as little more than criminals despite their official status. This internal conflict within the World Government forces expands on the political landscape for the series.
The Marines are known to possess some of the strongest individual fighters in the world of One Piece. Truly, the most powerful of the Marines must be those that include the current officers and the legendary ones from the past. People like Garp, Sengoku, and Zephyr have left their mark on history from the Marines and are still affecting things in the present storyline.