A literal interpretation of "After the Sea-Ship" will focus on the poem as a description of a ship's wake on the open ocean. Essentially, this is the entire content of the poem - a rather brief and ebulient description of how the water looks and acts in the wake of a ship that has passed.
The ship receives almost no description while the water is treated with characterization and specific, detailed description.
Symbolically, the poem is open to intepretation with the most likely and compelling interpretation being one which sees the ocean as a grand natural force, or even as a symbol for nature at large. The sea is not only undisturbed by the ship, emotionally speaking, the sea seems to take joy in the passing of the ship and to be characterized by great equanimity (evenness of temper).
To read further into the text, we might argue that the sea plays a role as background to life and so is symbolic of a temporal space within which life takes place. This space exists before life and also after life, with life being symbolized by the passing ship, and the sea then becomes associated with a temporal expanse that includes death. Death, however, is not dark and dreary but is instead, as the water is "frolicsome" after the passing of the ship, death is for Whitman a part of the wonder of existence.