Fleet

The World of One Piece is predominantly covered by water, as such the main mode of transportation is vis ships. Thus when an organization becomes strong or large enough, it may need to facilitate a Fleet in order to support it. Not many organizations have the funds or influence to support one, but if one is able to, they can exert a lot of influence across the Blue Sea.

Structure and Hierarchy
A fleet, unlike a normal alliance has a hierarchy, with one clear leader, with all the other ships' captains being subservient to them. The Marines are shown to have a very strict hierarchy, assigning ranks to each sailor, thus facilitating a chain of command, in the event the upper ranks are isolated or eliminated in battle.

Pirates, however, are much more laid back. If it's a single crew fleet, the most powerful and highest ranking combatants are generally put in charge of their own division (or ship); as such they follow the orders of the fleet "commodore" (or admiral) full time. With a multi-crew fleet, the subordinate crews form a "parent-child alliance" with a single "parent" and his crew ranking above the "children" crews. The captains of each crew are effectively division commanders in their own right. However in a multi-crew fleet, unless the fleet's leader orders it, the individual "children" crews are permitted to roam freely and act as they please, provided their actions don't hurt the "parent's" crew.

Marines

 * Fleet Leader: Fleet Admiral Sakazuki
 * Flag Ship: None
 * Status: Active

Possibly the largest fleet in the world, they act as the police of the World Government, and have bases in all the four Blue Seas and throughout the Grand Line. They are commanded by the Fleet Admiral.

Krieg Pirates

 * Fleet Leader: Krieg
 * Flag Ship: Dreadnaught Sabre
 * Status: Unknown

They were once one of the strongest forces in the East Blue, with 50 ships and 5,000 pirates to their name. However, they lost all their ships shortly after entering the Grand Line to the hands of Shichibukai Dracule Mihawk. It is unknown what has become of them after Monkey D. Luffy defeated their captain.

Baroque Works

 * Fleet Leader: Crocodile
 * Flag Ship: None known
 * Status: Imprisoned

This organization, which was once comprised of over 2,000 men and several ships, mainly dealt in bounty hunting and Underworld trading. The ultimate goal of Baroque Works was to be resolved in Operation Utopia, however several of their ships were destroyed by Portgas D. Ace, while the rest (after Crocodile's defeat) were captured by the Marines.

Blackbeard Pirates

 * Fleet Leader: Marshall D. Teach
 * Flag Ship: Unknown
 * Status: Active

Originally a small crew of only 10 or so members, shortly after the Battle of Marineford they swiftly took over the majority of Whitebeards territory, with Teach ascending to the ranks of Yonko. Now ruling over several islands, they have blossomed into a full fledged fleet capable of supervising them. Though the full size of the Blackbeard Pirates is unknown, they have at least ten ships to their name, with their division commanders are known as the Ten Titanic Captains (10人の巨漢船長).

Golden Lion Pirates

 * Fleet Leader: Shiki
 * Flag Ship: Island Ship
 * Status: Imprisoned

Known twenty-three years ago as one of the world's biggest pirate fleets, they attempted a bold plan of world domination, using the Ancient Weapons. However, after Gol D. Roger (who was the only one who knew the locations of said weapons) refused to join them, the fleet clashed against the Roger Pirates, in what is later known as the Battle of Edd War, and lost, incurring heavy damage. Shiki was later imprisoned after storming Marineford due to Roger's surrender, but later escaped. In One Piece Film: Strong World, Shiki gathered his fleet together again in order to act out his new plan of world domination; by using giant mutated animals to conquer his enemies, starting with the East Blue. However, after the fleet was defeated by the Straw Hat Pirates, they were then captured by the Marines.

Whitebeard Pirates

 * Fleet Leader: Edward Newgate
 * Flag Ship: Moby Dick
 * Status: Inactive

The Whitebeard Pirates were once a crew led by the Yonko, Edward Newgate, whose influence guarded and supervised several islands throughout the New World. The crew by itself has a large fleet comprised of 16 divisions, with over 1,600 crew members, but it is further reinforced by 43 allies, all seasoned veterans in the New World. This fleet made its appearance in the Battle of Marineford in the Marineford Arc. After losing their captain, it seems that the 'parent' crew have gone into hiding, while the 'children' crew are still active.

Straw Hat Grand Fleet

 * Fleet Leader: Monkey D. Luffy
 * Flag Ship: Thousand Sunny
 * Status: Active

The Straw Hat Grand Fleet was formed at the end of Dressrosa Arc, comprised of seven divisions: the Beautiful Pirates, the Barto Club, the Happo Navy, the XXX Gym Martial Arts Alliance, the Tonta Corps, the reborn Giant Warrior Pirates and the Yonta Maria Grand Fleet. An unusual fleet, Luffy initially turned down their offer for the alliance, as he finds such things as bothersome. After Luffy conveys to them that his ideal of Pirate King is not to be important, but to be free, the leaders were touched by this and decided to exercise their own free wills. They vowed themselves to be his underlings, without the Straw Hat captain having to be obligated to respond in turn.

Happo Navy

 * Fleet Leader: Sai
 * Flag Ship: Unknown
 * Status: Active

A large fleet in its own right, it has 1,000 members, it has been active long before the current storyline, with the former head, Don Chinjao, once being a rival to the hero Monkey D. Garp. This group appears to be based in the Kano Country, and have for generations been led by the Chinjao Family. They joined the Straw Hat Grand Fleet after their leaders were saved from enslavement as toys at the hands of the Donquixote Pirates.

Yonta Maria Grand Fleet

 * Fleet Leader: Orlumbus
 * Flag Ship: Yonta Maria
 * Status: Active

A massive fleet commanded by Orlumbus, comprised of 56 ships and 4,300 members. In a similar manner as the Happo Navy, Orlumbus too was turned into a toy and was doomed to a life of enslavement, but was saved by the Straw Hats. Due to this incident, he swore himself and his fleet to Monkey D. Luffy.

Neo Marines

 * Fleet Leader: Zephyr
 * Flag Ship: White Tiger
 * Status: Presumably Dismissed

The Neo Marines were an organization led by the former Marine Admiral, Zephyr, and was made up of a large fleet of ships. Their goal was to end the "Great Age of Pirates" by destroying the New World, and thus the legendary treasure One Piece which started this era. It is unknown what became of them after their defeat at the hands of the Straw Hat Pirates, and their leaders death at the hands of the Marines. They were the main antagonists of One Piece Film: Z.