And the startled little waves that leap In fiery ringlets from their sleep (3-4)
This description implies that the waves are somehow violent, pushy, or otherwise belligerent. Their "fiery" character even implies that they are potentially dangerous. The fact that the speaker is willing to sail through them suggests that he is willing to persevere.
As I gain the cove with pushing prow, And quench its speed i' the slushy sand (5-6)
These lines are similar to lines 3-4, in that the speaker appears to be fighting against something. The fact that he is "pushing" the prow" of his boat suggests a struggle of sorts. Clearly his effort pays off as he "gains" (which here means something like "reaches") his goal: the cove and the beach.
Then a mile of warm sea-scented beach; Three fields to cross till a farm appears (7-8)
These two lines have a certain list-like quality to them, one that is accentuated by the sequence of "and," "then," and "till" that starts in line 6. It seems kind of like a list of obstacles that the speaker must overcome, which points both to the difficulty of his task as well as his own drive to complete it.