The grey sea and the long black land; And the yellow half-moon large and low (1-2)
The natural world seems to be just sitting there at the beginning of the poem; its not clear what it's doing. The moon has been traditionally associated with women and the feminine (think Diana/Artemis from mythology being associated with the moon), and its pairing with the land and sea (things like farming and seafaring are traditional male pursuits) here suggests the coming together of opposite sexes that will happen at the end of the poem.
And the startled little waves that leapIn fiery ringlets from their sleep (3-4)
Waves aren't literally "fiery," and they don't literally "sleep" either. In a sense, the speaker figures the natural world – makes it do things it doesn't literally do – as the result of his passion. He's in love with someone, and going to meet her, and thus interprets the natural world in terms of his own "fiery" passion.
As I gain the cove with pushing prow, And quench its speed i' the slushy sand (5-6).
Nature seems to be two different things here. First, it offers a kind of resistance, in that the speaker must force his way with a "pushing prow." At the same time, the speaker reaches a "cove," which appears to offer a form of shelter, something nourishing and safe.