From Bulbapedia, the community-driven Pokémon encyclopedia.
This unnamed island is an anime-exclusive location that appeared in Beach Blank Out Blastoise!. It is located close to Cinnabar Island.
Ash and his friends first learned of the island after they bumped into a concerned Wartortle on one of Cinnabar Island's beaches. Upon arrival, the island's sandy shores were covered in Turtle Pokémon fast asleep inside their shells. Pikachu's Thunderbolt helped wake the wild Pokémon up, while Ash's Squirtle put on its infamous black sunglasses and took charge of the situation.
Brock was able to translate the Wartortle and Squirtles' story into a pictorial presentation which detailed that the Turtle Pokemon's King Blastoise went out swimming and later washed ashore asleep and before long the drowsiness spread to the other Pokémon. Jigglypuff was later revealed to be the cause after it became stuck inside one of Blastoise's cannons.
Team Rocket attempted to steal Blastoise several times with their mecha Gyarados, but were sent out to sea and their submarine exploded underwater.
The island is uninhabited by people and not marked on any map. The tropical location features a large rock formation that resembles a Squirtle shell, and its shoreline is dotted with palm trees.
Pokémon seen on the Unnamed island