Part of the success of this excellent poem is the way that the author uses meter to enact the sound and the rhythm and the feel of the waves as they lap against the boat of the speaker's imagination. Of course, the meter varies tremendously as you go through the poem, with the iambic pentameter at times changing to spondees that give these lines a definitely different feel. Consider the following example:
And the wheel's kick and the wind's song and the white sail's shaking...
The spondee of "wheel's kick" and "wind's song" and "white sail's" help to enact the sound of the waves of the sea hitting the side of the boat and the rhythm of the water. This is a technique that is used not just once in the poem, but in other lines. Now that I have identified this example try to scan lines 7 and 11 to analyse the use of spondees by the author. You might want to think about how dactyls are used as well. Good luck!