Conquerors Darkness
From the back cover:

The Earth has been transformed into a wtaery breeding ground for mutants and dreaded alien Star Beasts.  True humans are confined to island reservations or a life on the high seas.  Few ever escape the mundane life of island existence to join the mysterious Sea-Lords.

But one landsman, Dovirr Stargan, is determined to become a Sea-Lord, and he is willing to kill or die for the chance.  The brutal Star Beasts who devastated the Earth have been gone for many years, but the desire for vengeance is a living force among the Sea-Lords.  And when they do return to plant their eggs the Star Beasts must face the fury of the united forces of humanity... landsmen, Sea-Lords and mutants, and the new Ruler of the Nine Seas -- Dovirr Stargan.
Science fiction, Fiction, science fiction, general
Robert Silverberg
Robert Silverberg is an American author, best known for writing science fiction. He is a multiple winner of both the Hugo and Nebula Awards.

Silverberg received a Nebula award in 1986 for his novella Sailing to Byzantium, which takes its name from Yeats' poem; a Hugo in 1990 for Enter a Soldier. Later: Enter Another; and in 2004 he was named a Grand Master by the Science Fiction Writers of America.

([Source][1])


  [1]: http://en.wikipedia.org/wiki/Robert_Silverberg