Mer
Mer is a world very different from Earth, with a variety of its own creatures with varying degrees of intelligence, such as the monkey-bird and the leviathan. The world has twenty seas, and most of the crew's stops are made at islands. Parts of Mer are continually in flux, like a river of spiked rocks that rises out of the ocean in the first episode and appears to defy physics (none of the characters are bewildered by the spectacle aside from its danger), but whether this is the Dark Water's doing or just the nature of the geologically hyperactive planet is unknown.
Octopon was once the greatest city on Mer, referred to as "the jewel in the crown of Mer" by Ioz in episode 14, though it lies in ruin until Ren collects the first seven Treasures. It is then partially restored. Octopon seems to have been centuries ahead of the current technological state of Mer, although it is probable that civilization is continually in decline due to the Dark Water oozing from the planet's core.
Read more about this topic: The Pirates Of Dark Water