Watch Below — James White

Six humans survive a shipwreck, and find themselves in an airtight hold at the ocean's bottom. They have oxygen tanks, and stored food. They must survive in this cold, dark world until someone, somewhow, finds them.

And from out of space come the aliens, water-breathing aliens. Their own world has perished in a titanic heat-wave which boiled away their oceans. They had to survive, to reach another world, to find a new home.