I presume you are referring to Larkin's poem "The North Ship", which contains the line "Over the proud, unfruitful sea."

The poet describes watching three ships sailing by. The first ship turns west "by the wind was all possessed/And carried to a rich country." The second ship turns east "the wind hunted it like a beast/To anchor in captivity." The third ship doesn't sail at all but drives towards the north where there is no wind to take its sails.

Larkin continues to talk about the ship that drove towards the north in the next stanza, describing the sea it drives across as "proud" and "unfruitful"—meaning that the sea in the northern direction does not provide the sailors with help in getting to their destination. In fact, in the next stanza, he says the ship continues on its arduous journey long after the two others ships have sailed back into port.

The northern ship, he says, was "rigged for a long journey," but it is a long journey due to time rather than distance. At the same time, one feels the ship isn't going anywhere special. At least not like the other two ships. They may even have no real destination in mind, traveling "wide and far", as the poet says, with no set return date.