Every arc of One Piece has something in it to love and appreciate; however, when fans talk about which arc changed how they view One Piece, it always comes back to the Water 7 arc. What's so great about Water 7 is the way it sets up numerous plotlines -- even before the Straw Hats dock in the Venice-inspired island city -- and then pays it all off by the end in ways we couldn't imagine.
From the start, the Straw Hats motive for being there is clear: they want to use the treasure they got in Skypiea to repair their ship, The Going Merry. They also want to find a shipwright who can join their crew. While this seems like it'll be easy at first, these simple goals lead to various new plot threads that feel page-turningly engaging.
The Straw Hats encounter two very different groups during their exploration. One is the pride of Water 7, the Galley-La Company, an organization made up of hardworking but tough as nails shipwrights led by the company president and mayor of the city, Iceberg. The other is the Franky Family, a crass and scummy gang of ship dismantlers (who also work as bounty hunters) led by the most disliked person of Water 7, Franky.
The Straw Hats meet with Galley-La first, hoping to have their ship repaired, but this leads to some very upsetting news. After an inspection by shipwright Kaku, it's revealed that Merry's keel is badly damaged, which means trying to sail with it in its current state would be suicide, and trying to repair it would be mean making a new ship entirely from scratch. Luffy is given the option to either abandon Merry or ignore the shipwrights' advice and risk sinking out at sea. However, before a decision can be made, the Straw Hats get something else to deal with when Usopp, and the money he was carrying, is kidnapped by the Franky Family. Now, they must find their lost Nakama and reclaim their stolen money.
This leads to a confrontation between a severely beaten Usopp and Franky, who appears to be set up, at this point, as the main antagonist for this arc. Franky pities Usopp and smears Luffy's name, saying he must be ashamed to have such a weak crew member. This burns in Usopp as he tries to take down the gang leader, only to beaten down even more as Franky leaves with the Straw Hats' money. Eventually, Luffy, Chopper, Sanji and Zoro arrive on the scene to get revenge for their injured comrade by destroying the Franky family's home, but at this point, there's no recovering their stolen money. It's here that Luffy does something he doesn't normally do: make a difficult choice.
Even though Luffy's the captain, it's easy to forget that given his easy-going personality. He also relies heavily on his crewmates to do the things he can't. But this time, Luffy steps up and makes the tough call to move forward with a different ship. This leads to one of the most emotional moments in the entirety of One Piece.
While all of this is happening, Robin suddenly disappears. Creator Eiichiro Oda puts this on the back burner, though, while the Straw Hats' internal drama unfolds. Robin's disappearance comes back into play much later.
Once Usopp is in a stable condition and learns of Luffy's decision, he's vehemently against it. To Usopp, Going Merry is just as much a crewmember as the actual crew; to abandon her would be like abandoning an injured Nakama. This sparks a huge argument between the captain and the sniper, a scene completely out of the left field for One Piece. Even if they haven't always seen eye-to-eye, the Straw Hats have always been on the same page for the sake of their friendship, but now, Usopp and Luffy, arguably the two closest members, are in complete opposition to each other.
Usopp feels he's never been able to pull his weight the same as Zoro, Sanji or Luffy. He's not as strong and he's not brave, so if Going Merry gets replaced, what's to stop the Straw Hats from eventually replacing Usopp, as well? It's a difficult scene, punctuated with Sanji's line to Luffy -- "don't say something you'll regret" -- but the damage is already done.
A duel is decided between captain and sniper, with the winner claiming Going Merry. The fight is one of the best in One Piece, partly because of its strong choreography -- an aspect of One Piece that's always a bit on the weaker side. Usopp knows Luffy well and controls the fight from the start by setting traps, manipulating his captain's emotions and predicting what he'll do. Still, it's not enough to close the gap, and the fight ends with Usopp's body in even worse shape than ever.
A deep sadness washes over the crew as they realize things will never be the same now. Zoro comforts his sobbing captain the only way he knows how by reminding him "That's the burden of being a captain. Don't doubt yourself. If you start becoming unsure, who can we have faith in?" It's a devastating moment, and the wildest part is, we haven't even hit Water 7's climax yet.
The next day, the unsettling news comes that Iceberg has been shot in his home and Aqua Lagoona, a massive seasonal tsunami is expected to hit Water 7 later that night. The people of Water 7 are in turmoil as the pride of their town is on death's door and upon Iceberg waking up, it's revealed his assailant was none other than Nico Robin, who the audience would have forgotten by this point was MIA. Now the Straw Hats are public enemy number one on an island they can't sail away from. Luffy doesn't believe one of his crew would be capable of such a thing so, of course, he tries to confront Iceberg, only to be ambushed by a vengeance-seeking Franky.
Franky turns out to be a buff and confident but goofy guy who just so happens to be a cyborg. If this we're any other kind of shonen anime, it might feel out of place, but One Piece is a series with a talking reindeer doctor, gorilla salvagers and a corrupt king who eats cottages and cannons. A robo-man with a pompadour fits right in. Their fight is interrupted by the Galley-La shipwrights, who are also convinced it was the Straw Hats who attacked their boss. Luffy and Nami narrowly escape from them.
It's at this point that we realize that what started as a fun romp in a beautiful city of water has turned into an all-out manhunt for a fractured crew in just a single day.
Luffy and co. stakeout Iceberg's mansion in the hopes that Robin will return and they can question her. Eventually, Robin is confronted, as well as the real assailants, but the turnout is a truly unexpected one. Rob Lucci, Kaku, Kalifa and Blueno are all government agents from the intelligence organization CP9, sent to the city undercover to extract the blueprints for a superweapon from Iceberg. It's a very shocking, well-planned reveal, and one that's foreshadowed in just a few passing conversations. It truly makes the climactic final battles in Enies Lobby feel earned.
Water 7 juggles an internal dispute between Nakama, a murder mystery, government conspiracy and a natural disaster without ever feeling bloated. This is because Oda doesn't overload the reader with exposition. Despite this arc's length, it's exceptionally well-paced and fast-moving. The only exception to this is Franky's backstory; however, that just gives greater weight to his character's motivation, while also allowing us to explore the history of Water 7 and the creation of the Sea Train, The Puffing Tom.
It's still one of Oda's best arcs, and the climax at Enies Lobby further cements him as one of the best comic book storytellers of all time.