Seek'st thou the plashy brink Of weedy lake, or marge of river wide, Or where the rocking billows rise and sink On the chafed ocean side?
The speaker is back to talking to the waterfowl directly in the third stanza.
He asks the bird if he's seeking the "plashy brink" of a "weedy lake," or the "marge" (the edge) of a wide river, or the place where the "billows" (another word for waves) risk and sink near the "chafed ocean side."
As in the first stanza, there's a lot of description here. Things get pretty specific in a hurry. The speaker doesn't just ask if the bird is headed towards a lake, a river, or the ocean, but if he's headed toward a "plashy brink," that means edge of a place that's full of puddles, like edge of a lake.
The speaker doesn't really know where the bird is going, so he uses this moment of doubt to imagine some of the different places he could be going. (Albuquerque is out, we take it.)
Since the waterfowl could be headed towards a lake, or a river, or even the ocean, it is clear that the speaker isn't really sure what kind of bird he's dealing with.
The waterfowl is just some kind of generic water bird.
Since this bird is a generic bird, we get the sense that just maybe the speaker sees the waterfowl as some kind of symbol or metaphor for something else.
In other words, it's starting to seem as though "To a Waterfowl" isn't a natural or scientific poem describing a bird, but a poem about how the waterfowl helps the speaker see or realize some deeper spiritual truth.
Let's keep reading and see what else the speaker has to say about this waterfowl.