The quickest way to describe Terminal Island in the time that the Wakatsuki family lived there (without the dad, of course) is to say that it was a company town owned by the fishing companies.
The company owned everything, so things were not very nice. The homes they lived in were just little shacks made out of boards, for example. Jeanne's mom would have to go to work at whatever time of day the boats came in.
For Jeanne and her brother, it was kind of a scary place because so many people were strange to them. They had classmates from various different ethnic groups, and even the Japanese were from a lower class than they were.