About 30 years ago, I started reading a book, but never finished it.
There are several sets of characters, some who live on land and some who live underwater. I think the underwater people live in secret.
At some point, a couple's flying car has to land on water and they are rescued by a waste dumping ship. The water was so polluted it dissolved the paint from their car. One of them recognizes one of the mind-wiped slaves on the ship as her brother.
Underwater, there are nuclear waste containers leaking near their settlement, causing birth defects and death.