In this poem, William Butler Yeats describes a flock of swans on the water. He has observed these creatures for nineteen years running. This time he counts fifty-nine swans drifting together on the still water of a turlough (seasonal lake) at Coole Park, which is the estate of Lady Gregory, a wealthy Irish woman who patronized many writers in her day.
Although Yeats describes other sights in his poem—the trees and the swans themselves—he does not describe these as being reflected in the water. This is probably because of the time of day. It is twilight—not morning, midday, or afternoon. The sun has nearly set or has already set, so it would not allow for the reflections that would be seen at other times of day.
The only thing Yeats describes as being reflected on the water is "a still sky." This most likely describes a clear sky or a cloudy sky on a day with little wind. If there are clouds in the sky, they are not moving. Presumably the water is not moving, either, and the reflection of the still sky on the still water creates a very calming picture.