The speaker of "The Seafarer" is pretty darn lonely and miserable out there all by himself on the cold ocean. So why, then, does he spend most of his life there? Because his soul is overcome by a constant restlessness, an urge to travel that he just can't fight – that's why. His "heart's thoughts" urge him to travel, although there's a big part of him that just doesn't want to. It seems the only thing that can put an end to the Seafarer's discontent is an eternal life with God.
The speaker in "The Seafarer" feels torn between his desire for stability and security and his restless urge to travel.
According to our speaker, eternal life with God as the only thing that can bring an end to his dissatisfaction.