From Bulbapedia, the community-driven Pokémon encyclopedia.
Phantom gazing at the Sea Crown
Ash surrounded by a yellow aura
The Sea Crown (Japanese: 海の王冠) is an item of great value that resides in Samiya. Its foundation is a large stone, shaped similarly to a layered cake, with many light-blue crystals sticking out of it that can be removed. It is protected by a sealed door which can only be opened by the mark of the People of the Water. It is said to be the touchstone between the People of the Water and Water-type Pokémon, and whoever wears the crown is is known to be the "King of the Sea".
The Sea Crown is a mysterious piece of technology that appears to have multiple functions: It is a sort of "Engine" that propels the temple through the underwater depths, and also emits a containment bubble surrounding the Sea Temple that refracts light from the outside (rendering it invisible unless exposed to a lunar eclipse). The Sea Crown also maintains a breathable atmosphere on the inside, and controls and stabilizes the limitless amounts of incoming water that flows throughout the structure's aqueduct system.
During the events of Pokémon Ranger and the Temple of the Sea, Phantom, by removing one of its crystals which acts as the Crown's power source, compromised the containment bubble of the temple. This allowed the surrounding ocean to flow into the temple at an uncontrollable rate, causing the temple to stop moving, lose buoyancy, and begin sinking. Fortunately with the help of Jackie, Ash, and May, the jewels were returned to the Sea Crown, effectively restoring the containment bubble and saving the Sea Temple from irreversible destruction.
Ash becomes the "King of the Sea" by repairing the Sea Crown. Ash then becomes surrounded by a yellow aura, giving him the ability to breathe while under the water and swim through it rapidly. This power was also bestowed to people near him.