One Piece: Unlimited Cruise 1 - The Treasure Beneath the Waves is an action adventure game based on the One Piece anime and manga series. It is the first half of a story which is continued in the follow-up game One Piece: Unlimited Cruise 2 - Awakening of a Hero. The game starts with the Straw Hat crew being caught in a storm at sea while on the Sunny when they spot a glowing orb in the sky. After getting closer Luffy is sucked in the glowing orb and is told that he must visit the four islands of Dreams and face the ordeals on them to get a fitting reward. A strange creature called Gabri acompanies the crew as they face the ordeals.
In the game, the player takes control of the members of the Straw Hat crew. These are Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, Sanji, Tony Tony Chopper, Nico Robin, Franky and Brook. Various islands have to be explored and the player has to beat-up enemies including sub and end bosses. On the island, various items can be found that can either be directly used or crafted into something else. The player can also catch fish with a fishing rod, catch insects with a net and brake up rocks with a pickaxe.