Croatia

On the island of Dugi Otok, the furthest out to sea element of the triple island ring, west of Zadar, is the famous salt lake Mir.

An excerpt from the article 51 facts about Croatia
Its surface area is 0.23 km2, and its maximum depth is 5.8 m. The lake is connected with the sea by underwater channels. The lake's bottom is covered with mud with medicinal properties, and the water is much warmer than in the sea.