"To a Waterfowl" is addressed to a… waterfowl so, naturally, a waterfowl appears throughout the poem. This isn't, however, the poetic equivalent of a show on the Discovery Channel, and the waterfowl isn't just a bird. For the speaker of this poem, the waterfowl symbolizes something very profound: the existence of a "Power," a spirit, a.k.a. God, informing and directing the universe itself. Not only that, the waterfowl is also a double for the speaker himself. The speaker implies that he feels lost—that he is "floating" like the bird seems to do up in the sky above—but also comforts himself in the belief that he may only think he's lost. Like the waterfowl that appears to wander but is actually being guided, he too knows that God is looking out for him.
Lines 1-4: The speaker apostrophizes the waterfowl, asking it where it is going. It is described as "solitary," which foreshadows the speaker's implied solitude at the end of the poem.
Lines 7-8: The waterfowl slowly "floats along." While this seems innocent enough, it becomes clear that this little moment foreshadows the poem's later interest in wandering and being lost. Even at this early moment, we can see the waterfowl beginning to symbolize the speaker himself.
Lines 9-12: The speaker lets loose his descriptive powers as he again apostrophizes the waterfowl, asking about its potential destination. His questions reflect a deep uncertainty at the heart of his being about the order of the universe. He will be more explicit about this later, when he speaks of being lost and lonely, but at this point, he merely hints at it in his doubts about the waterfowl's own wandering, which of course mirror his own.
Lines 13-16: Well, it turns out that the waterfowl isn't "wandering" after all. It appears to be just floating through that desert that is the sky, but his path is in fact guided by a mysterious "Power" ("teaches thy course" is a metaphor for this guidance).
Lines 17-20: The waterfowl is now a symbol of endurance. He flaps and flaps and flaps his wings all day, up there in the cold, thin air of the upper atmosphere. He refuses the comforts of the "welcome land," even though a very ominous "dark night" is near. That charming little phrase is almost certainly a metaphor for the "dark night" of death.
Lines 21-24: Lots of anaphora (the repetition of "Soon") and alliteration (all those S sounds) appear in this stanza (check out "Sound Check" for more on that). We also can't help thinking that "toil" is a metaphor for life, and toil's "end" yet another metaphor for—wait for it— death. Sure, the speaker hasn't switched gears and started talking about himself yet, but he soon will. In some ways, this stanza is the speaker's indirect, almost euphemistic way, of talking about the "rest" (another metaphor) that is death (as in "Rest in Peace").
Lines 25-26: All the subtle references to death in the preceding stanzas now give way to the very dark lines that begin second to last stanza: "abyss of heaven." Heaven isn't really an abyss, so this is a metaphor for the way in which the waterfowl has suddenly disappeared somewhere in the sky. The fact that the speaker is now all alone, so to speak (there is no waterfowl for him to look at) foreshadows the poem's final stanza, in which he talks about a time later in life where he will need to go it alone. We also kind of think the waterfowl needs to be symbolically killed off here, so that the speaker can focus on how this whole experience relates to his own life and stop obsessing over the bird's flight through the heavens.
Lines 29-30: Okay, wait just a second here. The waterfowl's flight is now "certain"? This implies that it is regular, normal, guided. What happened to all that floating about from earlier? We know that the bird is the same. At the same time, the speaker has changed, and so has his perspective. He realizes that the waterfowl is, in fact, not wandering or floating at all, but following a very regular, well-defined path managed by God.