Summertide

Set more than four thousand years in the future, Summertide introduces a galaxy widely populated by humans and a variety of intelligent aliens, all of whom live in the shadow of the vanished race known only as the Builders. Nothing is known about the Builders, but the gigantic artifacts they have left behind - many of them still hardly understood - dominate the areas of space in which they are found.
One such is the double-planet system of Opal and Quake - the former covered in water, the latter in desert - connected by a Builder device called the Umbilical. It is to this system that a variety of humans and aliens come, ostensibly to witness Summertide - the annual tidal wave which sweeps across Opal.