From Bulbapedia, the community-driven Pokémon encyclopedia.
The Marine Cup Tournament (Japanese: マリンカップトーナメント Marine Cup Tournament) is a competition in the anime that is held on a cruise ship traveling through the Decolore Islands. It is a 16-competitor tournament consisting of single Pokémon battles that was featured in Cilan and the Case of the Purrloin Witness!.
It was used by a criminal named Teaque to cover up a theft he committed. In the final round of the tournament, Teaque battled Ash and had his Galvantula temporally shut down the electricity on the cruise ship with Thunderbolt. No one accused Teaque of doing this, since it seemed like Galvantula's Thunderbolt just missed its target. With the electricity down, Teaque's Vanillite was able to steal Mrs. Ripple's "Eye of Liepard" jewel. Meanwhile, Ash defeated Teaque and won the tournament. Teaque was later unveiled as a thief and arrested.
Pokémon used in tournament