The speaker says that his feet are "cast in icy bands, bound with frost, / with frozen chains." He conflates the harshness of the cold weather with being chained. He is illustrating his feeling that he is bound to this lonely life at sea. Not only is it cold and lonely; he feels like he is forced (chained) into this way of life. The image and feeling of coldness is likened to feelings of sadness and loneliness. The cold weather isolates him from the warmth of human contact and from the comfort of life on land. He says life at sea is a "world blown clear of love" and "hung with icicles."
Each time returning to sea, the seafarer feels, is accompanied by the blackening of night and the onslaught of harsh, cold weather. Whether or not he is not exaggerating about the cold weather, the point is that he associates coldness and darkness with being alone at sea. In contrast, he imbues images of life on land with warmth and human company.
Although he speaks of his life at sea as being chained or forced, the seafarer does go to see voluntarily and he takes pride in bearing this burden. When he begins speaking of his need to go to sea (despite the cold, lonely experience of it), he stops using these images of cold weather. This is significant. So, at first it shows that he associates his feelings of loneliness with the cold. But when he speaks of his need to go to sea, his tone changes. Thus, in thinking of his life at sea as a sacrifice, his colors his time at sea with the phrase "the joys of God / Are fervent with life." And from this transition, he moves on to thoughts of heaven.