The People of the Water depicted in an ancient tapestry.
The People of the Water (水の民) are an ancient civilization whose existence predates that of modern society. Also, their technology was considered to be revolutionary due to the fact that they created the Sea Temple Samiya as a homage to honor the Manaphy species. The descendants of the People of the Water protect a rare Manaphy Egg, which holds the key to discovering the location of their ancestors' temple.
The crystals in resonance with a lock.
The People of the Water are also known to possess a variety of special crystals, which are able to unlock the entrance to the Sea Crown's location. Each member of the current tribe are known to wear the crystals as necklaces. When the crystals are held in close proximity to the tablet describing the Sea Crown, both gems will resonate and a huge doorway will materialize from nowhere.
The last surviving individuals linked to the People of the Water are Lizabeth, Kyle, Ship and Meredith, whom all possess tanned skin and black hair. To combat thieves finding the entrance to the Sea Temple, the People of the Water constructed elaborate traps to prevent them from stealing the coveted Crown.
Trivia
The civilization of the People of the Water bears many resemblances to the "lost" civilization of Atlantis, in which both societies possessed greater technologies than our own, and were lost to beneath the ocean depths.