Umigakure no Sato

The Village Hidden in the Sea is a village located 160 meters under the sea. Before the massacre, this village was famous for having some of the best Water Release user's and its preference of peace and not interacting with those on the surface.

Mark of the Sea
The Mark of the Sea is an ancient Juinjutsu that the founders of the Village Hidden in Tides created and passed down to the first kage. Anyone born into this village is receives this Juinjutsu the moment they are born, placed upon them by the current kage. It's easy to recognize those marked with this Juinjutsu as their skin is covered with a nearly transparent film-like substance. This substance reacts to light in a peculiar way as the light cases the individuals skin to look like the scales of a fish of it is looked at closely.

This substance has only one purpose: to give the user the ability to live thousands of leagues under the sea just as naturally as they would live on land by giving them a unique form of immunity to any kind of water. Among many other things this immunity allows them to easily breathe under water, be immune to all effects of water pressure, see in the dark, be empowered while in the water, the ability to drink any kind of water they want without any ill effects, etc. When individuals with this Juinjutsu emerge from the water, the film-like substance immediately absorbs all the excess water to empower itself. This protects the user against any ill effects that would befall after emerging from the sea upon living in the water for most of their lives. The substance also helps the body keep in moisture when the person is on land. It is so effective that not even Scorch Release or very high temperatures can evaporate or effect the water that is inside the body.

This trait hides a dark secret about the Juinjutsu, a secret that only the members of the Watasumi and Hozuki Clan know of due to their sensitivity to water. While nothing can cause the water in the substance and the individual's body to disappear, even the most severe of temperatures, water still seems to disappear after awhile when the person has had no contact with seawater. For those in the Hozuki and Watasumi Clan this phenomena usually doesn't happen until a month of no contact with normal seawater while it takes only a week for this phenomena to happen for other individuals. When the water starts to disappear the individual will feel out of breath, in pain, and very weak. If the individual doesn't come into contact with normal seawater in a day, then they will die and experience tremendous pain as they do so. The only evidence that this phenomenon is occurring is how the individual's skin will look like it is breaking apart. In other words, this substance forces any civilian to stay in the sea.

Most people who leave the village for good will die because most of them do not know about the secret, which explains why the village had very few anbu among their ranks. The only way to prevent this from happening is to splash normal seawater on one's body periodically so the film-like substance will always have enough water.

Trivia

 * This village was heavily inspired by the people and sea village in the anime: Nagi no Asukara.
 * Some people, like Otohime and Hoori, are born with the Juinjutsu. This makes it impossible for anyone to do stuff like: remove, tamper, effect, or influence it in any way, shape, or form because it becomes a natural ability rather than a simple Juinjutsu.