But now they drift on the still water, Mysterious, beautiful (25-26)
The speaker describes nature as something we cannot fully understand. We know that it is beautiful, but it is still mysterious. In other words, it isn't like a math problem that we can quite "solve." Perhaps the mystery of the swans partly accounts for their power over the speaker's psyche.
[…] when I awake some day To find they have flown away? (29-30)
Here we get a sense of the potential alienation of man from nature. The speaker imagines the swans leaving him again, flying away and setting up shop somewhere else. It seems that the swans, like the rest of nature, will outlast him in the end. Sad.