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With reference to the stagnant, stale or foul water which collects in a ditch, the phrase (as) dull as ditchwater means: very boring or unexciting.

The later phrase (as) dull as dishwater (which refers to the greasy water in which dishes have been washed) is probably due to mispronunciation of the noun ditchwater in the original phrase.

These are, in chronological order the earliest occurrences of the phrase (as) dull as ditchwater that I have found:

1-: From an extract from the draft of the first issue of a periodical named Hernan’s Miscellany, written in 1770 * by the Irish playwright and politician Richard Brinsley Sheridan (1751-1816)—as published in Memoirs of the Life of Right Honourable Richard Brinsley Sheridan (London: Printed for Longman, Hurst, Rees, Orme, Brown, and Green, 1825), by the Irish author Thomas Moore (1779-1852) [Vol. 1, chapter 1, page 30]:

“In examining the beginning of the Spectators, &c. I find they are all written by a society.—Now I profess to write all myself, though I acknowledge that, on account of a weakness in my eyes, I have got some under-strappers who are to write the poetry, &c. . . . . In order to find the different merits of these my subalterns, I stipulated with them that they should let me feed them as I would. This they consented to do, and it is surprising to think what different effects diet has on the writers. The same, who after having been fed two days upon artichokes produced as pretty a copy of verses as ever I saw, on beef was as dull as ditch-water * * * *”

* This draft is dated 1770 in Thomas Moore’s book.

2-: From the diary of the Irish republican and rebel Theobald Wolfe Tone (1763-1798)—as published in Memoirs of Theobald Wolfe Tone (London: Henry Colburn, 1827) [Vol. 1, page 398]:

[15^th June 1796] Got a parcel of English newspapers from Sullivan. Strolled out into the fields, all alone, and lay down under a hedge to read them. […] Reading these papers has left me as dull as ditch water, and I did not need that.

3-: From a letter to the Editor, presenting a new translation of works by the Roman poet Horace, published in The Spirit of the Public Journals for 1797. Being an impartial Selection of the most exquisite Essays and Jeux d’Esprits [sic], principally Prose, that appear in the Newspapers and other Publications (London: Printed for James Ridgway, 1802) [page 147]—reprinted from an undated issue of The Morning Chronicle (London, England):

It [i.e., my translation] is not heavy as lead, like Mr. ——; nor dull as ditch-water, like Anna Matilda; nor mad as a March hare, like our present excellent Laureat; nor stupid—— but I should never make an end, if I went on with my comparisons.

4-: From Squibs and Handbills relating to a Mayor-Choosing at Beverley; now first collected and reprinted as they were originally published ([s.l.]: [s.n.], 1824) [No. 8. Great News!!!, page 12]:

“The vile, beastly, infernal, knavish, diabolical faction that opposes me, has undoubtedly got all the wit and satire on its side: we are al [sic] as dull as ditch-water; for, excepting a blockhead of a Shoemaker, who unites bad grammar and Blue Paper, there is not a soul amongst us that could fill half a sheet of paper, to save his life.”

5-: From Goslington Shadow: A Romance of the Nineteenth Century (New York: Published by Collins and Hannay, Collins and Co., E. Bliss and E. White, and W. E. Gilley, 1825), by Mungo Coultershoggle [Vol. 1, chapter 10, page 112]:

For want of better, owing to the high duty on malt and hops, we are obliged to swill down beer as thick as puddle, and dull as ditch water.

6-: From The Humble Petition of Common Sense to the Editor of the Journal, by ‘Touchstone’, published in The Cumberland Pacquet, and Ware’s Whitehaven Advertiser (Whitehaven, Cumberland, England) of Tuesday 30^th January 1827 [page 3, column 2]:

It may be enough
To tell you plainly that your head, they say,
Is made of most impenetrable stuff!
Thick as a block, and hard as lignum vitæ!
Dull as ditch water!—see, Sir, how they slight ye!

These are, in chronological order the earliest occurrences of the phrase (as) dull as dishwater that I have found:

1-: From The Boston Morning Post (Boston, Massachusetts, USA) of Monday 16^th April 1832 [page 2, column 2]:

Consolation to Blockheads.—Though the most celebrated personages have generally given, in early youth, the earnest of future greatness, yet, on the contrary, there have been instances of great abilities, that gave no promise of distinction till near the meridian of life. Robert of Sicily, distinguished for bravery and genius, was in his youth as torpid as a polar bear; Claude, the artist, was as dull as dish-water; La Fontaine shew no signs of rhyme till his twenty-second year; nor Dryden till he was twenty-seven; and Cowper did not become an author till fifty.

2-: From the Alexandria Gazette (Alexandria, Virginia, USA) of Monday 21^st July 1834 [page 3, column 1]:

Harvard College.—One of the Boston papers states that the public exercises at Cambridge, on Monday, was “a flat business altogether—as dull as dishwater!”