Ena is what the first original humans had to live under the sea. It's compared to as being like fish scales. Ena out of the water is like normal skin except it will glitter and shimmer lightly in the sun. Those with Ena are extremely sensitive to UV light/sun rays when out of water. If they're out of water for a long period of time, they can faint and the Ena will slowly chip away like glass. Some can gain Ena, such as Tsumugu and Miuna but seemingly only if they have people of the sea in their family lineage.
Since the sea god's power was decreasing, the people of the sea had to get ready for hibernation, in order for him to gain his powers back. The people from the sea had to eat to their heart's content until they weren't able to eat anymore. Their ena kept getting thicker, which meant that their Ena gets much drier than before. With the thick Ena, they were able to hibernate. But only if they were in the sea since Chisaki stayed on the surface after the Ofunehiki and did not hibernate.