Thalassocracy
A thalassocracy is a state whose realms are primarily marine; an empire at sea, such as the Thracian and Phoenician, the latter being based on land within the borders of the cities of Tyre, Sidon and Carthage.The term (a combination of the Greek "thalassa", "sea", and "kratos", "rule" or "government") was first used by ancient Greeks to describe the government of Minoan civilization, whose power depended on its navy.
More modern examples include Venice and Dubrovnik. The Portuguese Empire and the British Empire also started as thalassocracies, but they eventually acquired large land territories.