You are correct. The idiomatic phrase is "water under the bridge," which means
"Something that happened in the past and cannot now be changed."
From: http://dictionary.cambridge.org/us/dictionary/english/water-under-the-bridge
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.