Summary: It drifts towards the surface, neither fully animal nor fully machine.
It drifts towards the surface, neither fully animal nor fully machine. Its mouth filters the microplastics that saturate the waves, much like the filter feeders of prior centuries. Its belly is full of microbial fuel cells that turn the plastics into fuel and feedstock for itself. Even if damaged, it is able to self-repair from ambient materials. Healing is not solely the domain of the organic anymore.
It does not travel alone, but instead moves in a pod. These cyborg creatures are large, whale-like submarines able to power themselves from both solar power and oceanic plastics. Their exteriors, however, are coated in biological matter, intending to serve as mobile niches for marine species. They are from a drowned island country's attempt at ecological tourism of now-dead coral reefs, but the machines outlive those who built them. While the reefs are dead, the species that once dwelt their follow alongside the pod.
The machines are able to adapt their self-repair to make entire replacement parts for new bodies, allowing them to slowly increase their numbers. The whales are threatened by scrap pirates, mechanical poachers, and illegal salvagers, so they possess similar habits to their biological forebears. They travel in numbers and submerge when human watercraft approach. They know the instincts of cetaceans, for their programming was ultimately based on the mind of a long-dead organic whale.
They are but one of several species of hybrid terraforming machines built and released into nature, to re-terraform the planet after centuries of extractive industry and pollution. Others fill the stratosphere, restoring the damaged ozone layer. Others crawl across ravaged wasteland, consuming industrial waste like a buffet. The humanity's detritus powers its creations, like the drifter. Even a dead world may re-emerge to life, although the clades may change.