This poem makes use of a great deal of imagery—vivid descriptions of sensory information—in order to really bring the experience of the speaker to life and to show, perhaps, how vibrant the world appears to one who is involved in a romance. The speaker describes the "grey sea and the long black land" as well as the "yellow half-moon large and low"; both of these are visual images, describing something one might see. The speaker describes the "pushing prow" of the boat and how its speed is slowed by the "slushy sand": both of these are tactile images, describing something one might physically (as opposed to emotionally) feel.
In the poem's second part, after the speaker's boat has landed, the speaker describes the stretch of "warm sea-scented beach," an image that combines both the tactile and the olfactory, something we might physically feel and something we could smell, respectively. Adding an image that addresses the way the place not only looks and feels but also smells makes our mental picture that much more vivid. The sense of smell is supposed to be the most linked to memory, so if the reader has ever smelled the warm beach, such an image is going to be quite effective in terms of conveying experience and bringing the reader emotionally closer to the story told by this poem.