The speaker's deep desire, as the title of the poem suggests, is to go back to sailing the seas in a boat. The sea is in his blood: he has a fever for it.
The speaker says he wants a tall ship. Unlike others who might want a ship to kill a whale (like Captain Ahab), to find a treasure, to explore new places, to make a fortune, or to find a new home to settle in, the speaker simply wants to be on the sea for the sake of being there. He longs for the sea breezes, the gray mists, the wind, the white clouds, and the sound of the seagulls crying. He yearns for this freeing environment, where he can live the "vagrant gypsy life" untethered from the shore.
To the speaker, the seafaring life is a simple life where one can find happiness in the natural world, in telling stories with a friend (a "fellow rover"), or in sleeping and dreaming sweetly.