A reference to Alfred Tennyson's poem Morte D'Arthur:

Morte D'Arthur, last verse:
So said [Arthur], and the barge with oar and sail
Moved from the brink, like some full-breasted swan
That, fluting a wild carol ere her death,
Ruffles her pure cold plume, and takes the flood
With swarthy webs. Long stood Sir Bedivere
Revolving many memories, till the hull
Looked one black dot against the verge of dawn,
And on the mere the wailing died away.