pre-noun adjectival
of the ocean; in the open sea; oceanic
Poetic pre-noun adjective used in classical or literary contexts to describe something as being of the sea or on the open sea. Often appears in waka poetry and set phrases.
White waves rise on the open sea.
I can hear the cries of seabirds out at sea.
Alternate kanji spelling; 津 is an old character for 'port' or 'harbor', but here it is used phonetically.
The つ is an old possessive particle (equivalent to modern の), so 沖つ literally means 'of the offshore'. The form is preserved in classical poetry and some place names.