I think "sleep with the fishes" is related to the mafia. It means when someone has been murdered by the mafia and they've dumped the body into the ocean - so they're sleeping with the fishes. I don't think you would use it for a regular death, it's quite specific.
Some more that I know are:
"Pushing up daisies", meaning someone is dead and buried. "Dead as a dodo" or "Dead as a doornail", meaning definitely dead. "Croak", which means die. I think this is an Australian one. You use it like "He croaked".