Those ancient creatures astonishingly evolved from land dwellers into marine organisms, and as for the Undead, needless to say, they don't rely on oxygen to survive, so after the flood disaster struck, they continued to move freely under water.
The Bug Race had even less to worry about; their rate of evolution surpassed that of the ancient creatures, and they also emulated humans by developing biological warships. With these biological warships, the Bug Race could not only create many underwater members, but also preserved a portion of their flying contingent.
