maritime nation
A country whose identity, economy, or security is closely tied to the sea; often used in geopolitical or historical contexts.
Japan has traditionally been regarded as a maritime nation.
海洋国家 stresses a nation's maritime, seafaring identity, whereas 島国 simply denotes the geographic fact of being composed of islands.
Compound of 海洋 (ocean) and 国家 (nation). The term is a modern geopolitical concept, likely calqued from Western 'maritime nation' or 'sea power'.