A Short, Sharp Shock — Kim Stanley Robinson

A man tumbles through wild surf, half drowned, to collapse on a moonlit beach. When he regains consciousness, he has no memory of who he is or where he came from.
He knows only that the woman who washed ashore with him has disappeared sometime in the night, and that he has awakened in a surreal landscape of savage beauty--a mysterious watery world encircled by a thin spine of land. Aided by strange tribesmen, he will journey to the cove of the spine kings, a brutal race that has enslaved the woman and several of the tribesmen. That is only the beginning of his quest, as he struggles to find his identity in this wondrous and cruel land--and seeks out the woman whose hold on his imagination is both unfathomable and unshakable.