Devour and Evolve

Tyler Barton is Victor Bennett's vice captain, and he is the one who kills him. Victor is an SS rank esper, and the killing happens in a world where flooding has covered everything and sea creatures have changed into animals that hunt people. Victor comes back as a small carp with almost no life in it. In that state he wakes a system that lets him devour and evolve, and with it the bloodline of the ancient Cosmic Fish. He eats monsters and climbs the ranks. He gets an investigator named Ava Goodwin out of danger, meets his old teammate Neil Carter again, and the three of them go after whoever set the flood in motion.