From Bulbapedia, the community-driven Pokémon encyclopedia.
Bomba Island (Japanese: バンバ島 Bamba Island) is an anime-only location in Hoenn. It was featured in the episode Judgment Day!. It's close to Lilycove City. The Pokémon Battle Judge Training Institute, a place where people can train to be referees, can be found here. The island is circle-shaped and seems to have good grass.
It is possible to reach the island by ferry from Lilycove City. Ash, Brock, May and Max ended up on Bomba Island by taking this ferry by accident; they were in a hurry and thought they were on the ferry they wanted, the one to Mossdeep City. While there, they met Jimmy and his teacher Serena and learned that referees were trained on the island.
Even though Jimmy was being trained on the island to become a referee, it was not what he originally wanted to do; he ended up on the island by accident while he fell asleep in a boat that drifted away. After his Wartortle evolved to a Blastoise during a fight with Team Rocket who he then defeated, Jimmy decided to stop his referee training and leave the island.
Pokémon seen at Bomba Island
Its name may come from the Greek word βόμβα, which means bomb.