Unfortunately very little, as they keep very much to themselves and are so few in numbers and so quick to move from place to place that it’s difficult to keep track of them.

It is known that they’re a highly nomadic people whose origins are a matter of strong debate. Some stories say that they’re from Sinnoh, as what’s known about their lore seems to center around Manaphy. Others say they’re from Kanto, as their technology bears striking similarities to the tools of Pokémopolis, Pokélantis, and other extinct Kantonian cultures. There are even claims that they’ve come from Alola, Hoenn, Unova, and more than one region in the South Pacific. Perhaps the most creative theory is that they’re remnants of the lost continent of either Atlantis or Mu. However, as interesting as that theory is, it seems to only be supported by fringe conspiracy theorists on the internet. I would say that the most mainstream theory is the one that proposes the People of the Water are from Kanto, as that’s the one with the most compelling argument.

Regardless of where they’re from, what’s known about them is that the People of the Water are actually a collection of small, nomadic tribes scattered throughout the globe. Many take on odd jobs in the towns and cities they pass through, but some take on professions centered around water, such as one particularly notable tribe that formed a rather famous water circus.

Samiya, meanwhile, was built as a temple for the Manaphy. Stories conflict concerning whether or not the People of the Water ever used it as a place of residence—or, in other words, we’re not sure if it was ever a city for the People of the Water or even a part of one. It’s simply their best-kept secret, a floating fortress that travels from one ocean to the next, completely undetectable by even modern equipment. (It’s part of the reason why we’re not even sure if it actually exists, although we do indeed have one report of its sighting in recent times, from one of Professor Oak’s own proteges.) The Sea Crown, the People’s holiest object, is actually the item that powers Samiya and ensures it remains afloat. Supposedly, it has some sort of power over water-type pokémon as well, but again, this is merely a rumor, rather than confirmed fact.

In short, I’m afraid at most, all I have are rumors and very sparse scraps of information. It would certainly be wonderful to know more—especially if the People of the Water are connected to Manaphy—but unfortunately, unlike the Draconids, they’re not likely to divulge anytime soon.