The colloquial phrase will a duck swim? (also would a duck swim?, does a duck swim?) expresses enthusiastic acceptance or confirmation.

This phrase occurred, for example, in an interview of the Irish businessman Mark Fielding, published in the Sunday Independent (Dublin, County Dublin, Ireland) of Sunday 6^th August 2006 [Business section: page 4, column 3]:

Has your bank, building society or another financial services company ever overcharged you?
Does a duck swim? Within two years, my new bank had overcharged me in excess of £2,000. To demonstrate the bank’s audacious attitude and to add insult to injury they asked if they could repay me over three years. My answer is not for publishing.

The earliest occurrences that I have found of the phrase will a duck swim? (also would a duck swim?, does a duck swim?) are as follows, in chronological order:

1-: From The History and Fall of Caius Marius. A Tragedy. As it is Acted at the Duke’s Theatre (London: Printed for Thomas Flesher, 1680), by the English playwright Thomas Otway (1652-1685) [page 33]:

Nurse. Henceforward do your Messages your self.
Lavin. Nay, prithee be not angry, Nurse, I meant
            No ill. Speak kindly, will my Marius come?
Nurse. Will he? will a Duck swim?
Lavin. Then he will come.
Nurse. Come? why, he will come upon all four, but he’ll come.

2-: From Tit For Tat, a comedy in three acts (London: Printed for C. Dilly, 1788), by the English playwright and theatre manager George Colman (1762-1836) [Act I, page 17]:

Old Meanwell. Will you take any refreshment after your journey?
Skipwell. “Will a duck swim?”—I never refuses to crack a bottle with an honest fellow, for I love to wet my whistle.

3-: From The Review; Or, The Wags of Windsor. A musical farce, in two acts: As it is performed at the Theatres Royal, London and Dublin with the greatest applause (Dublin: Printed by Thomas Burnside, 1801), by the English playwright and theatre manager George Colman (1762-1836) [Act I, scene 2, page 10]:

Enter Martha.
Mar. Here’s a man, sir, come after the footman’s place.
Bull. I hope he’s civiller than the last fellow. Does he look modest?
Mar. Oh! yes, sir; he’s an Irishman!
[…]
Enter Looney Mactoulter.
Bull. So you want a place?
Loon. You may see that, with your own ugly mouth.
Bull. My ugly mouth! you have been in service before?
Loon. Does a duck swim.
Bull. Who have you lived with?
Loon. I have lived with the Mactoulter’s these nineteen years.

4-: From Forest of Montalbano: A Novel (London: Printed for George Robinson, 1810), by the British novelist Catherine Cuthbertson (c.1775-1842) [Vol. 2, chapter 22, page 424]:

Angelina […] eagerly availed herself of the first favourable opportunity of arising to retire, under the fair pretence of not delaying further the arrangements of the good priest for his departure.—
“But you will write to me, reverend father, I hope;” she kindly added.
“Will a duck swim, my darling?—Is it write to you, a vurneen¹? why sure that’s what I will, if it was for nothing but to be telling you about the comforts you showered on uncle Pat.”

¹ Cf. the phrase Kathleen Mavourneen.

5-: From Rosabella: Or, A Mother’s Marriage (London: Printed for Baldwin, Cradock, and Joy, 1817), by the British novelist Catherine Cuthbertson (c.1775-1842) [Vol. 4, chapter 10, page 169]:

“You see, Miss Acres,” said Captain O’Dowd, gravely, “[…] Miss Frederick will be taken to Paris, if she will condescend to go.”
“Condescend!!” exclaimed Miss Acres, contemptuously. “Condescend indeed!! Well, that is a monstrous good one! Will a duck swim?”
“As assuredly as a magpie will chatter,” said Mrs. O’Dowd; “so I trust my two young ducklings will be allowed to swim across the channel with the protecting duck and drake of the brood of the O’Dowds, as soon as the senate of grave talking birds have discussed the propriety of granting permission.”

6-: From Beauchamp; Or, The Wheel of Fortune (London: Printed for A. K. Newman and Co., 1817), by James Holroyd Fielding [Vol. 4, chapter 8, page 134]:

“You will meet them, lady Earlingbury, to-morrow week?”
“Will a duck swim? Do you suppose I am capable of missing so delectable a treat?”

7-: From Love, Law, and Physic: A farce, in two acts. As performed at the Theatres Royal (Dublin: Printed by T. Charles, 1821), by the English playwright James Kenney (1780-1849) [Act II, scene 4, page 34]:

Doc. Come then, Mr. Log, since you are so resigned, you’ll be a guest at the wedding dinner, at any rate.
Log. Will a duck swim;—great fortunes ai’nt to be got at, every day; but as for pretty ladies—don’t I see ’em all about me?

8-: From A Collection of Proverbs, and Proverbial Phrases, in the Persian and Hindoostanee Languages (Calcutta: Printed at the Hindoostanee Press, 1824), by the Scottish army officer and lexicographer Thomas Roebuck (1781-1819) [Part I: Persian, Section II, No. 129, page 87]:

They asked a blindman what do you want?
He said the sight of my two eyes.

Spoken in reply to one who asks another if he will accept something which it is well known must be gratifying to him. (Eng.) Will a duck swim?

9-: From Caprice: Or Anecdotes of the Listowel Family. An Irish novel, in three volumes. By an Unknown (London: Sherwood, Jones and Co., 1824) [Vol. 2, chapter 20, page 299]:

“Has her ladyship accepted his offer?”
“Ha! that’s a good one! Will a duck swim? Will an elegant lady refuse such a fine clever man, with such fine property, and an earldom into the bargain?”

10-: From The Disagreeable Surprise; A musical farce, in two acts (London: John Cumberland, 1826), by the English author George Daniel (1789-1864) [Act I, scene 3, page 28]:

“Pray, Mr. Floor’em, may I propound a question?—Hav’nt you been to school?”—“Does a duck swim?”—“And what might you study?”—“Knox’s (Knocks’s) Essays! Will you have a taste?”

11-: From The Illustrious Stranger, or Married and Buried; An operatic farce, in two acts, as performed at the Theatre Royal, Drury Lane (London: Published by William Kenneth, 1827), by the English playwright James Kenney (1780-1849) [Act II, scene 5, page 47]:
—Context: Benjamin Bowbell, shipwrecked on an island off the coast of Malabar, has married Princess Irza, the daughter of Aboulifar, King of the island; in the following passage, Bowbell is about to be buried alive with his dead wife, when Azan offers to take his place:

Azan. King Aboulifar, your daughter’s death is attributed to me—I cannot survive the imputation—lead me to her dear remains!—proud in terminating with her a degraded existence.
Aboulifar. Our laws permit it, and you may claim them;—but you, my son-in-law, is it possible you would renounce the honour?—
Bowbell. Will a duck swim—will the Polly ² go to smash—will Sukey ² snap at me when I gets back to Cripplegate?
Aboulifar. Then you are unworthy of my blood!

² Sukey (diminutive of the feminine name Susan) is a child’s name for a tea-kettle. This may have originated in the nursery rhyme Polly put the kettle on.