From Bulbapedia, the community-driven Pokémon encyclopedia.
The crystal mark of the People activating a sealed door
The People of the Water (Japanese: 水の民 People in the Water) were a race of people in the Pokémon anime that were mentioned in Pokémon Ranger and the Temple of the Sea. Very little is known of their culture or their civilization.
As their name implies, the People were connected to the ocean, either living near or in it. They are apparently an almost extinct clan, with Lizabeth, Ship, and Lizabeth's parents Kyle and Meredith as the only known remaining descendants. They are characterized as having dark skin color and various shades of blue or black hair.
The People of the Water possessed surprisingly advanced technology, having made Samiya and the Sea Crown. Special crystals they own can activate these technologies, from opening doors, to keeping the Sea Temple from sinking. During the events of the movie, May was given a crystal mark from Lizabeth.
The Phantom appears to be very knowledgeable about the People of the Water and even obtained one of their special crystals prior to the events of the movie.