Why do we say "A Fish Out of Water"?

Meaning:

Someone who is uncomfortable in an unfamiliar situation

Background:

This expression dates back to at least the 14th century, given that it can be found in the general prologue to Chaucer's The Canterbury Tales, written between 1387 and 1400.

The rule of Maurus or Saint Benedict,
By reason it was somewhat old and strict,
This same monk let such old things slowly pace
And followed new-world manners in their place.
He gave for that text not a plucked hen
Which holds that hunters are not holy men;
Nor that a monk, when he is cloisterless,
Is like unto a fish that's waterless;
That is to say, a monk out of his cloister.
But this same text he held not worth an oyster;...

(translation courtesy of librarius.com)

A similar sentiment is found in other languages but expressed differently. For example, speakinglatino.com offers Como sapo de otro pozo (like a toad from another well) and como cucaracha en baile de gallinas (like a cockroach in a chicken dance).

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