You are correct. The idiomatic phrase is "water under the bridge," which means
"Something that happened in the past and cannot now be changed."
You are also correct that it is common to misquote common expressions in comedies to sound stupid for comedic effect.
You might also want to Google "malapropisms" to find other amusing examples, such as these:
"He had to use a fire distinguisher.
Dad says the monster is just a pigment of my imagination.
Isn't that an expensive pendulum round that man's neck?
Good punctuation means not to be late.
He's a wolf in cheap clothing.
Michelangelo painted the Sixteenth Chapel.
My sister has extra-century perception.
"Don't" is a contraption."
All of these are from http://www.fun-with-words.com/malapropisms.html