Ena allows a person to breathe and swim underwater,
all sea people are descendants of the Sea God.
Ena is a special sort of skin, which allows the user to breathe in water, often when their skin cracks, they need to get in water and preferably salt walter.
It's shown that someone, who has been born from either a parent who came from the Sea can get Ena. This is shown as Miuna's mother was from the Sea and Tsumugu's grandfather as well.
It was first believed that they took Manaka's Ena who was dwarling in the sea, but later, this was said to be untrue and that this was because of someone in their family being from the sea, it was coincidentally that they got their Ena while falling into the sea.
This happens once the surface is about to have an Ice Age.
At that time, there's a specific date for an evening to eat to your hearts content, which after, you can't eat until the hibernation so your Ena can thicken and eventually fall asleep.
At some point, they'll wake up.