From the introduction:
The Worldsea is a vast untamed ocean. The characters' homeland is a tiny little niche within it. Over the last century or so, it had been a place of plenty. Plankton and seafloor vegetation flourished and so then did all the creatures which live on it. Food was plentiful little was needed beyond what home provided. In living memory no one has gone more than a full day's swim from home. But in the last five years the light from above has dwindled somewhat and the plankton and other plants have become more scarce. The other wildlife has begun to scatter in search of better grounds.
The people of home are not yet starving. But the worried mutterings have begun as many are not eating as well as before. In the process of feeding themselves, now the limited food sources remaining dwindle even further. The leaders have chosen a small group of younglings to explore further than any have before. To find more food and to perhaps seek a new home, if one can be found which does not suffer as home does.