From Bulbapedia, the community-driven Pokémon encyclopedia.
Wailmer Island (Japanese: ホエルコ島 Hoeruko Island) is an anime-exclusive island somewhere off the coast of Hoenn. It is located somewhere between Slateport City and Ever Grande City.
Surrounded by dangerous whirlpools, it does not officially appear on any maps, but it has been visited by a number of humans, and it has one known inhabitant, Robin. Additionally, a group of Wailmer live around it. Aside from Robin, other humans who are known to have visited the island include Ash, May, Max, Brock, Jessie, and James. It appeared in the anime episode Island Time.
Pokémon seen on Wailmer Island