Nights into Dreams Wiki

You see that black sea down there? Make sure you stay away from it.
~ NiGHTS, NiGHTS: Journey of Dreams.

The Dark Ocean is a dangerous ocean with black water located behind the Dream Gate in NiGHTS: Journey of Dreams. According to NiGHTS, the Dark Ocean can destroy a visitor's soul.

NiGHTS: Journey of Dreams

During the tutorial of both Will and Helen stories, NiGHTS and the visitors are having fun flying around the Dream Gate, when they fly above the Dark Ocean. NiGHTS and Owl introduce the Dark Ocean to the visitors, and explain to them that they shall never go near it because they may never return.

Later when both Will and Helen recover three out of four Ideyas, Wizeman attacks NiGHTS and Helen at the Dream Gate and drags both into the Dark Ocean, promising to reset NiGHTS by replacing their mind and soul with a more obedient one. Will watches all of this and decides to jump into the Ocean to save NiGHTS and Helen.

Will is so determined to save NiGHTS, that his red Ideya of courage grants him with the power of flight and Will manages to escape from the Ocean. Meanwhile, Helen somehow escapes from Wizeman and is also granted with the power of flight by her Ideya, thus, escaping from the Ocean as well.

In the secret ending, Wizeman captures and drags NiGHTS into the Ocean, but not Helen. Instead, he takes both Will and Helen's yellow Ideya. Will and Helen jump into the Ocean and are granted with the power of flight by their red Ideya and escape from the Ocean.