at sea (in a broad sense that could also include (large) lakes – see sjö) · to sea (in a broad sense, like above)
köken: From till (“to”) + sjöss (form of sjös (“sea's”) that only appears in this expression), with frozen genitive from when till governed the genitive.
köken: From Old Danish til syøs, i.e. the preposition til (“to”) (originally taking the genitive case) + the noun sø (“sea”). Compare Norwegian Bokmål til sjøs and Swedish till sjöss.