Fleet

A Fleet (船団) or Armada (艦隊) is a seafaring organization that is strong or large enough to facilitate a large number of ships to support the main transportation of said organization. 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 admiral (or "commodore" in the English dub) 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" or "sibling's" crews.

In the world of One Piece, if a fleet belongs to a country but is independent of the Marines and the World Government, they are generally labelled as a Navy (水軍). They are technically not considered pirates, and thus not illegal (as Sai's presence was allowed at Mary Geoise during the Reverie) but at the same time are not a part of and do not possess the universal authority of the Marines, and as such, may come into conflict with them.

Navigation
Flotta pirata Flota Flota