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.
沖つ白波が立つ。
White waves rise on the open sea.
沖つ鳥の鳴く声が聞こえる。
I can hear the cries of seabirds out at sea.
沖 is the common noun for 'offshore' or 'open sea', while 沖つ is a poetic, archaic pre-noun form.
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.