Wiktionary
sleep with the fishes
vb. (context idiomatic English) To be dead as a result of being murdered; usually also means the corpse has been dumped in a body of water.
Usage examples of "sleep with the fishes".
I thought about asking him if he wanted to sleep with the fishes, but Chow was looking antsy, so I refrained.
At least a third of the lizards would finish the crossing, and all or nearly all of his selkies would sleep with the fishes when it was over.