Complex sentence: Practically all of the sea's islands are either in coastal waters or belong to the various islands making up the Kuril Islands chain which fall either under undisputed Japanese or Russian ownership or disputed ownership between Japan and Russia.
Split into simpler sentences: Practically all of the sea's islands are either in coastal waters or belong to the various islands making up the Kuril Islands chain. These fall either under undisputed Japanese or Russian ownership or disputed ownership between Japan and Russia.