The Infinite Sea — Jeffrey A. Carver

Since John Bandicut saved Earth from collision with a comet in Neptune Crossing and saved Shipworld from the boojum of Strange Attractors, his life has become one confrontation with chaos after another.

No sooner has he survived a harrowing escape from Shipworld than he and his eclectic band of aliens find themselves struggling to survive beneath the waves of a distant ocean world. They quickly discover they are not alone. A race of deep-sea humanoid amphibians, the Neri, are in trouble of their own. Long under attack by beings from the surface, they now face an even greater peril from the seafloor depths, a powerful force known as the Maw of the Abyss.

If Bandicut cannot solve the riddle of the Maw, the Neri's beleaguered world will die. And Bandicut and his friends will die with it.