Cormorants have relatively wettable plumage, which reduces buoyancy and helps them pursue prey underwater, and most species spread their wings after diving. The Galapagos species kept the posture even though its wings are short, ragged and useless for flight.
The flightlessness itself has a genetic basis that researchers traced to changes in genes affecting limb and cilia development, several of them the same genes implicated in human skeletal dwarfism. Isolation on islands with no native mammalian predators removed the selective pressure to keep costly flight muscle, and the bird reallocated toward powerful legs and dense bones for diving.
The population has always been small, in the region of a couple of thousand birds, and confined to two islands. That concentration is the real risk: an oil spill, an El Nino food collapse or the arrival of rats or cats on those coasts could affect a large share of the world population at once.