From Bulbapedia, the community-driven Pokémon encyclopedia.
Maisie Island (Japanese: レフレ島 Refle Island) is an anime-exclusive location in Hoenn found between Lilycove City and Mossdeep City. The island is also nicknamed "Clamperl Island" because of the many Clamperl that live nearby.
It appeared in Clamperl of Wisdom, when Ash and his friends met up with Spoink again. After Spoink lost its pink pearl in the surrounding waters, a search of the island ensued. The group eventually reunited Spoink with its pink pearl, and the Clamperl research laboratory had its rare blue pearl returned, after it was taken by Spoink in its desperation to have a pearl.
Isaiah lives on Maisie Island, where he runs his own Clamperl research laboratory, which is complete with a pool enclosure home for several of the Bivalve Pokémon. Many young Trainers like Madeleine also live on the island.
Pokémon seen on and around Maisie Island