Trying to remember a book title

[rolex]

It's a sci-fi novel. I read it about 40 years ago. A freighter is torpedoed in a convoy during the second world war. Some of the crew survive in an air pocket and live inside the sunken ship for several decades utilising the supplies on board. They're eventually rescued by aquatic aliens seeking to colonise earth's oceans after there own planet becomes inhabitable.

Edited by rolex on Sunday 25th May 16:03