Answer:
A morbidly-named body of water
What has two spines and a lot of ribs, and carries much but never moves?
Walk on the living, they don’t even mumble. Walk on the dead, they mutter and grumble
To cross the water I'm the way, for water I'm above. I touch it not and, truth to say, I neither swim nor move.
I drift forever with the current down these long canals they’ve made, Tame, yet wild, I run elusive Multitasking to your aid. Before I came, the world was darker, colder, sometimes, rougher, true. But though I might make living easy, I’m good at killing people too.
Often cooked as fillet, this meat comes from an animal that swims underwater.