This really is a full grown larva: it's so fat and heavy it's not able to float beneath the water surface anymore and can only in shallow water it's able to get the tail tip to the surface to get some air. At this stage there's some danger of drowning when in an aquarium. It does not eat anymore and soon will crawl out of the water to dig a little pupation hole in the banks near the water where it will pupate to become the adult beetle. I tried to persuade him to do this in a terrarium outdoors, but it had its own ideas about things and managed to escape. A few weeks later we sitting outside at night a Great diving beetle flew with a clang against our oil lamp . Who knows, could be this specimen. It flew away and we did not see it again.