The Watch Below

The Watch Below (1966) is a science fiction novel by British writer James White about a colony of humans stranded underwater in a sunken ship, who survive by air pockets, and a water-breathing alien species in search of a new home. The two generation ships encounter each other in the Earth's ocean.
Reception.
Algis Budrys of "Galaxy Science Fiction" liked the novel, stating that it was the first generation ship story to expand on Robert Heinlein's "Orphans of the Sky".