as secret as a fish

[chameleon2]

i came across this expression,and from the context i can tell that it means some one whose real intentions are always kept hidden...
would you please tell me if i'm right ? and how the fish might be secret !!!!!!!!

[David L.]

Scrooge never gave away any money. He never told
anyone anything. He was as secret as a fish. He had a
cold heart and an unkind face, a pointed nose, red eyes
and thin blue lips.

A Christmas Carol
Charles Dickens

[chameleon2]

Scrooge never gave away any money. He never told
anyone anything. He was as secret as a fish. He had a
cold heart and an unkind face, a pointed nose, red eyes
and thin blue lips.

A Christmas Carol
Charles Dickens

thank you

but the second part of the question is still unanswered ..
how the fish might be secret ???

[Anglika]

Fishes can't speak, so they can't tell a secret.