Dreams of the Sea — Elisabeth Vonarburg

As the twin planets of Altair eclipse each other, a mysterious blue Sea rises up from the darkness, killing most of the Earth colonists who have settled the strange new world. In the devastating aftermath, the survivors must make a new life amid the abandoned, yet curiously intact cities of a long-vanished alien population. But their greatest challenge is yet to appear, for in a world of ancient memory and forgotten history, the answer to salvation may lie in the strange dreams of the colonists themselves. But are their dreams the key to survival, or a final warning of the dangers to come?

So begins Book 1 in the epic of Tyranael, a five book series from one of Canada's most beloved Science Fiction authors.