I have seen a lot of farewell e-mails from people taking a job at a different company in which they say “So long and thanks for all the fish.” Ha ha, they’re leaving and they mention the Hitchhiker's Guide to the Galaxy. Funny, right?
Only if you don’t think about it at all. [The following might sound like a spoiler but it’s not; it’s from like the first few pages of the book.] The dolphins use that phrase as their final farewell after leaving Earth. They have tried to warn the humans of the impending destruction of the planet, but the humans didn’t listen and haven’t sensed the danger. So the sentiment behind that message is “You are doomed. I’m kind of sad about it because you're nice, but I’m not going down with you.”
Not exactly the sentiment you want to convey to your former co-workers when switching jobs.