From Bulbapedia, the community-driven Pokémon encyclopedia.
The Great Sea Spirit Sapphire, better known as simply the Sea Spirit (Japanese: 海のたましい), is a magical scepter that enables the user to communicate with Water Pokémon. The sapphire is a traditional part of Whirl Islands lore, and is a vital part of the Whirl Cup Competition. It has been passed down through many generations of Sea Priestesses, such as Maya.
Its properties of communicating with water Pokémon are similar to the Sea Crown.