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.