The Watch Below

In The Watch Below, two narrative lines dovetail cleverly. In one a World War Two merchant vessel sinks, leaving three men and two women to survive in a large air pocket, work out life-maintenance systems and eventually breed there Under the Sea while 100 years pass. In the other, water-dwelling Aliens, who have long been seeking a wet world like Earth to inhabit peacefully, land their Generation Starship in the sea in time to save the descendants of the five twentieth-century survivors.