"dull as ditchwater" vs. "dull as dishwater"?

[Joel S. Berson]

Can the learned members elucidate:

When did these two phrases come in to use?

If "dishwater" was later, why?

When using Google Ngrams, must I enclose the phrases in quotation
marks, or does Ngrams treat an unquoted sequence of words (before a
comma) as a phrase and not individual words?

Why does Ngrams tell me "ditchwater" arose around 1827 and
"dishwater"  around 1875 when I find one instance of the former
between 1700 and 1799 and none of the latter until perhaps the mid-1900s?

Can anyone explain the flawed and inconsistent Google Book search
results when a time period is specified?  E.g., if I ask for "1700 -
1799", I find "dull as ditchwater" at 1747; if I ask for "1600 -
1799", GBooks says none.  Or if I ask for "any time - 1910", GBooks
alleges that it appears in the Manifesto of the Communist Party (and
no other book)?

Joel

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