The Land of the Sea (海の国, Umi no Kuni) is featured in episode 169 through episode 173. It consists of several islands, Mother Island being the main one. It's surrounded by three other islands; Jiro, Taro, and Demon Islands. Jiro Island is where Anko Mitarashi was abandoned and taken as a test subject for the Cursed Seal of Heaven by Orochimaru, and was eventually found by the Anbu.
Orochimaru had a hideout here, and several of his forbidden experiments were conducted here as well. One of his subordinates continued the work after Orochimaru abandoned it, leading to attacks on the local ships. It is not a prosperous nation, and relies on shinobi from Kirigakure for protection. Fifteen years after the Fourth Shinobi World War, the Land of the Sea has attempted to expand its influence, causing friction with the Land of Water, to the point the Land of Water is uneasy with signing a treaty that would give the Land of the Sea more stability.