One thing we can say for sure about the speaker of "To a Waterfowl" is that he has an eye for detail, an eye for nature. In the first stanza of the poem, he can't ask where the waterfowl is going without launching into a description of the "falling dew," the glow of the heavens, and how this glow appears like a series of "rosy depths." Later on, he draws upon his observations of various aquatic environments to talk about a "weedy lake," a "river wide," and a "chafed ocean side." Any way you slice it folks, the speaker knows the natural world in all its detail and never shies away from telling us about it. In his own words, nature's images are always "deeply" etched in his heart.
Now, if you've read our "Detailed Summary," you know that the speaker doesn't just love nature for its own sake. On the contrary, he's one of those guys who likes to find meaning in the world around him. The speaker is a spiritual man, and for him the natural world is the place to go for proof of the existence of some spiritual power that is responsible for the order and function of the universe. This is what he means when he talks about that "Power" that guides the waterfowl's seemingly aimless flight, or when he talks about a nondescript "He" that will guide his "steps."
Okay, so why all this emphasis on the existence of some pervasive spirit that can be found in the natural world? Is the speaker an evangelist, committed to teaching his readers deep spiritual truths? To some extent, yes, but we think there's a little more at stake here.
The second half of the poem is full of metaphors. The references to the "dark night," a "sheltered nest," an end to toil, and to being swallowed up are all metaphors for death. Clearly, death is on the speaker's mind, which is a little… odd. He doesn't come across as terribly anxious or worried or anything like that, but he's clearly a little worried about walking through life lost and alone.
It's almost like the speaker is on the cusp of adulthood, at a point in life where he realizes that he wants the universe to have a spiritual backbone. He wants there to be meaning in the world, and he wants to know that even if he feels lost and alone, he will never actually be totally lost and alone. He wants to believe that there is an order to the universe, a plan for his life.
That's why he seems at peace after the fourth stanza, when he comes up on—for lack of a better way to put it—the existence of God (he doesn't call the "Power" that, but we will, just for convenience). That knowledge tends to put him at ease. Like the waterfowl, he knows has a GPS (or at least a compass) to guide him through life.