The Adventures of the Water-Knight
Kay had this peculiarity, that his breath lasted nine days and nine nights under water, and he could exist nine days and nine nights without sleep. . . . And he had another peculiarity -- so great was the heat of his nature that when it rained hardest, whatever he carried remained dry for a hand-breath above and a hand-breath below his hand; and when his companions were coldest, it was to them as fuel with which to light their fire.
From the Tale of Kilhwch and Olwen.

Down, down, where the light is green and blue, deep down in the under-sea;
Through tangled forests where no birds sing, but fish swim silently;
Past coral castles that arch and spire, where the blue-haired sea-folk dwell;
Past old sea-gardens, dim as a dream, o'ergrown with weed and shell;

Down, down, to the wide wet pasture-lands, where the mild sea-cows graze
(Faintly their bells ring up through the sea, as they wander the sandy ways);
And on to the lonesome, weedy wastes that border the deeps unknown,
Where, silent and slow and ceaselessly, the tides march up and down.

Through that unknown world, 'neath the blue sea-roof, swam the Wondering Boy and Kay,
Kay the Knight, who for nine full suns in the watery world could stay;
And whatever he carried for light or warmth or food in his charmèd hand,
For a hand-breadth over and underneath was as dry as if borne on land.

Armored from head to foot was Kay, like a great fish silver-scaled.
On many a quest had he set forth, and never a quest had failed.
But never a quest like this before! The earth was filled with despair,
For the old sea-dragon, so long aasleep, had sprung from his secret lair.
From the gem-lit caverns the sea-folk loved, he forced them all to flee,
And strewn in glittering, wave-swept heaps lay the cities of the sea.
From coast to coast had the dragon raged, still proof against mortal might;
Till quick to the cry of the Wondering Boy came the valiant Water-Knight.

Now a sea-horse passed them, wild with fear, his white mane streaming back;
And now a bevy of little fish, with their eyes agog, in his track;
Then a murmurous music drifted by, like the song of a shore-bound shell,
And a group of little sea-maids fled past, waving a white farewell.

On the verge of the lower seas they stood; and before they plunged below,
Kay kindled the silver lamp he bore, which burned with a steady glow.
Far up through the watery dark they gazed, then dived through the deep once more,
Till they came to a long gray shape of dread that lay on the ocean floor.

"Now challenge him fair!" cried the Water-Knight, "as an Englishman must do.
No knight may creep up on his foe by stealth who would keep his honor true."
"Come out!" cried the Boy. "We are Englishmen!" They stood as a shining mark.
The answer came with a hissing sound -- a bolt, shot out of the dark.

"My fay!" cried the Knight, in sudden wrath. "Now hold up the lantern high.
Since this is the only tongue he speaks, we will make him a like reply."
Swiftly he hurled his faery lance, and leaped to the monster's side;
While the Boy held the silver lantern high, and the light spread fair and wide.

The bolts shot out, and the bright steel flashed, and ever its aim was true;
But harmless it glanced from the dragon's side, ere back to the Knight it flew.
"Is he proof against faery steel?" asked Kay, as his strength was overborne.
The faery lamp gave a sudden flare and flashed on the dragon's horn --
The single, towering magical horn that grew on the monster's brow.
Straight to that mark the lance went true, and the dragon was vanquished now:
A dumb and sightless and coward thing, he rolled on the ocean bed,
While swift through the seas, from rock to cave, the wonderful tidings spread.

The sea-folk builded their walls again to the music of singing strings;
While, thronging along the ocean paths, danced jubilant, finny things.
The mer-children played by the dragon's side, and wove him a seaweed crown,
As he lay, a helpless and harmless thing, where the tides march up and down.