The speaker yearns for life of adventure, exploration, and freedom. The term “wanderlust” best describes this feeling. It means the desire to travel and explore the world: “lusting to wander.” The speaker longs for the excitement of such wandering travel especially if the destination is uncertain. This yearning is also motivated by the traditional romance associated with the sea, it’s openness to possibility, the constant flux of the tides and the grandeur of it.
Metaphorically, this could be about life itself. Faced with the monotony of life, the speaker might be hoping for some adventure to come his way. He also may be trying to motivate himself to make a change in his life, to interrupt the monotony or to live more freely. He compares the trick (steering a ship) with the gull’s way, the whale’s way and the wind: completely free.
But he definitely indicates that there are downsides. The sea is lonely. The sky is grey. The wind is like a “whetted knife.” He wants a “a quiet sleep and a sweet dream when the long trick’s over.” He wants adventure but knows it will be difficult. He also eventually wants to come back to his current life.
One of the most memorable phrases for me from this excellent poem is when the speaker yearns in the final stanza to go back to "the vagrant gypsy life." He is a man that has obviously spent a long time on the sea as a sailor, and now that he is living on land, he finds himself caught up in a desperate desire to live on the sea again and experience the same itinerant, wild lifestyle that does not chain you to any specific location. He wants the same excitement and closeness to the sea and nature that such a lifestyle would give him. Note the first two lines of the last stanza:
I must go down to the seas again, to the vagrant gypsy life,
To the gull's way and the whale's way where the wind's like a whetted knife...
This then is the kind of life he wants: a life that is spent following the path of the gull, the whale and the wind, and a life that is "vagrant" and not stationary.