Essentially, Dickinson is comparing the bird's smooth flight to rowing in the ocean, saying that its strokes of the oars (which symbolize the bird's wings) that "divide the ocean" would be too delicate and "too silver for a seam," or a fissure, to be visible.
The poem's final lines (19-20) make this comparison clearer. In the preceding lines, 16-18, she translates both the movement (flapping → rowing) and the environment (sky → sea) into what's ultimately a straightforward marine metaphor. In the next lines, it's a bit different.
Or Butterflies, off Banks of Noon,
Leap, plashless as they swim.
She casts the butterfly's flight in directly aqueous terms. In her poem, butterflies do not flutter, they swim. They don't fly, they "leap." They don't set out from leaves or flowers, they set out from "banks."* And their movements are not merely serene, they are "plashless."
And, of course, the bird's movements are even "softer" than these butterflies and the oars.
This works really well because the sky and the sea are both vast, colorless expanses through which many things moved in many different ways.
ㅤ* Yes, the Banks are of Noon, but she still describes time using a water metaphor.