The "Water 7 Saga" (ウォーターセブン編 Wōtā Sebun Hen?) (Also known as the "World Government Saga" or "CP9 Saga") is the fourth saga in One Piece. After the adventure in Skypiea, the crew lands safely back in the Blue Sea, only to find themselves confronting Foxy the Silver Fox in the Davy Back Fight, and soon after, Aokiji, one of the three Marine Admirals. Narrowly escaping Aokiji, the Straw Hat Pirates reach Water 7, a city of carpenters, where Luffy is on a quest to find a shipwright for the crew.
They soon fall into trouble though, making enemies with both the Franky Family: a ship dismantling group, and the Galley-La Company: a shipbuilding company which employs some of the top carpenters in the world. They also learn that the Going Merry is dying, and because of this, Luffy decides to get a new ship. Usopp disagrees with the decision and leaves the crew. Meanwhile, CP9 of the World Government captures Robin by her will and flees to Enies Lobby, while the crew attempts to get Robin back. Finally, they get her free and get a new crewmate: Franky, who is their shipwright. As they are leaving, Usopp apologizes and is welcomed back into the crew, and so the Straw Hat Pirates are ready to set sail with a brand new ship and shipwright, Thousand Sunny and Franky, along with Robin and Usopp back in the crew.
- Long Ring Long Land Arc: (Manga: 2004) (Anime: 2004-2005)
- Water 7 Arc: (Manga: 2004-2005) (Anime: 2005-2006)
- Enies Lobby Arc: (Manga: 2005-2006) (Anime: 2006-2007)
- Post-Enies Lobby Arc: (Manga: 2006-2007) (Anime: 2007)
- This is the second longest saga in the manga, consisting of 139 chapters in 15 volumes. The largest is the Pirate Alliance Saga, with over 140 chapters.
- This is the only saga in the first half of One Piece to not introduce a new Shichibukai.
- It is during this saga that the Straw Hats' need for a shipwright, as well as a new ship to replace the Going Merry, becomes clear. This saga is the last saga to feature the Going Merry and was the first saga to feature the Merry's successor, the Thousand Sunny.
- This saga involves several members of the Straw Hats leaving the crew temporarily. During the Long Ring Long Land Arc, Chopper is forced to leave the crew during the Davy Back Fight between the Straw Hats and the Foxy Pirates (in the anime, Robin is forced to leave during the Davy Back Fight as well), though the Straw Hats later win him (and Robin in the anime) back. During the Water 7 Arc, Usopp leaves the crew following his and Luffy's fight over the Going Merry, while Robin is forced to leave due to the threat CP9 and the Buster Call pose to the Straw Hats. Robin later rejoins the crew during the Enies Lobby Arc, while Usopp officially rejoins (as he used his Sogeking alter ego during the later part of the Water 7 Arc and the whole of the Enies Lobby Arc) during the Post-Enies Lobby Arc.
- The Marine Admirals were first introduced in this saga, though only Aokiji and Akainu (during Robin's flashback of the Ohara Incident) actually appear. Kizaru is only mentioned briefly by Robin (he would later appear during the Sabaody Archipelago Arc in the Summit War Saga).
- Robin and Franky's back histories are revealed during this saga and their potential to revive the Ancient Weapons, Pluton is explained.