I built a one chunk fish/bone/ink sac farm in the ocean with water source blocks above a magma cube layer. Under that I have a water-logged hopper minecart running on a (complete coverage) rail layout. When water mobs spawn, get sucked down, and die on the magma cube layer, their drops just bob above the magma cubes and mostly do not get sucked up by the hopper minecart. I got no bones on my last afk session and not nearly the number of fish that I was expecting. I then jumped into the farm and ran around and collected nearly a stack of bones and fish.