Unoccupied seas
From Supremacy Wiki
Unoccupied Seas
Seas or oceans with no troops or companies in are available to move into and claim (without attacking).
If a sea contains an active company but no navies an attack move hast to be made to occupy this territory. It is not neutral so resources must be expended in trying to occupy it.
Pirates
Any troops that exist in sea territories which are unowned by a superpower are considered to be Pirates. They will occupy the territory, but will not attack.