Like its predecessor, the game is a fairly standard action-adventure game. You must take the Straw Hats through several islands filled with enemies to defeat, items to collect, and bosses to defeat. Said bosses and enemies are clones of the crew's old friends and foes, created from the Straw Hats' memories. You can level up the crew's moves and make them learn more by repeatedly using them in battle, and you can use the stuff you collect to have Usopp craft all kinds of bells and whistles, Chopper create healing and damaging items, and Sanji cook up various meals that enhance the crew's life and SP bars. There's also a Versus mode, where you can fight team battles against another player or the CPU; you can even play as the other characters and standard enemies in that mode.
Unlike its predecessor and successor, this entry is divided into two games: One Piece: Unlimited Cruise 1: The Treasure Beneath the Waves, and One Piece: Unlimited Cruise 2: The Awakening of a Hero. Both of them are rich enough to work as stand-alone games, but they work best as a pair, as not only do you then get the whole story, but you can also bring over your characters from Episode 1 to use them in Episode 2. The game was released in Europe and Japan, but Japan also got a compilation of both episodes for the 3DS, called One Piece: Unlimited Cruise SP, which also includes Marineford Mode, which is a recreation of the manga's Marineford Arc, adds most of the important characters from that arc as playable characters, and retweaks Whitebeard's moveset to match the manga. In Europe, the games were separated again, with SP only including Episode 1 and Marineford mode, leading to Episode 2 getting released seperately as One Piece: Unlimited Cruise SP2.
- Bait-and-Switch Boss: In Episode 2, Bellamy initially pops out of the first seed on the fourth island. But just as you expect a fight against him, Doflamingo comes out and chucks him away.
- Butt-Monkey: Usopp, as usual, gets the short end of the stick in several cutscenes.
- Continuity Cavalcade: There's a whole lot of miscellaneous references to the manga, ranging from attacks to quotes, pieces of dialogue, the bosses' introductions, etc.
- Defeat Means Playable: You can play as the bosses and Mooks you've defeated in Versus mode.
- Dual Boss: Doflamingo isn't fought alone, but with one of the other Straw Hats, who he controls with his powers. Not only does that make him more difficult, but it also means that unless you have items to revive them, they're pretty much doomed; you obviously can't select them when Doflamingo controls them, and he KOs them instantly when he releases them.
- Fighting Your Friend: A few examples.
- Some of the clones the Straw Hats have to fight are of their friends, like Monster Chopper, Nightmare Luffy and Ace in Episode 1, and Vivi, Paulie and Mr. 2 in Episode 2.
- Doflamingo can force the Straw Hats to fight at his side with his powers.
- The Final Boss is a transformed Gabri.
- Hub Level: The Thousand Sunny, which is extremely faithfully recreated.
- New Game+: After you beat an episode, you can replay it on a higher difficulty while keeping most of the stuff you unlocked and obtained in the first playthrough. It's in fact the only way to fully level up the Straw Hats' techniques.
- Old Save Bonus: You can bring over your characters and progress from Episode 1 to Episode 2.
- One Game for the Price of Two: Double subverted. Both episodes have enough content to last you quite a while gameplay-wise, but you must still get both entries if you want to know the whole story. The Japanese version of Unlimited Cruise SP doesn't have that problem, though.
- Pokémon Speak: Gabri can initially only say his name; in fact, the Straw Hats named him that precisely because he kept repeating it.
- Sequential Boss: Episode 1 has a fight against the cowardly Spandam and a bunch of Marine flunkies, followed immediately by a fight against Aokiji.
- Episode 2 has one of these for its final fight. After defeating Yami's first form (a large golemlike creature), he transforms into a much nastier creature composed of tree roots and darkness.
- Shown Their Work: The Thousand Sunny is extremely faithful to the blueprints Oda drew for it in the manga, right down to minor details.