The world of One Piece is clearly fictional and it doesn't resemble our world's Earth. This way the author gets a lot of freedom to create his own world.
The world of one Piece consists are three different kinds of environments in which the people live in. The Blue Sea, which is a huge ocean that covers the world. The next one is the sky ocean, which is an environment in the sky. Finally the third environment is the ocean floor.
The Blue Sea
The Blue Sea consists of one large continent, one long chain of islands, and four seas that make up two larger oceans. The continent is called the Red Line, the big chain of islands is called the Grand Line and the four seas are called North Blue, South Blue, East Blue and West Blue.
As explained above, North Blue, South Blue, East Blue and West Blue make up the four seas of the world. Within each Blue, sea travel works as it does in the real world. You can sail to any destination you want with a regular compass. However getting from one Blue to another is much more difficult. The reason is because not only the Red Line separates the two oceans, but also because the seas within one ocean are separated by a large sea route called the Grand Line as well as two Calm Belts.
Grand Line
It is the one sea where normal compasses fail. It is the destination of every pirate and the path that leads to unimaginable adventures. Most of the story is based in the Grand Line. The reason this sea is sought by every pirate, at least once in his life, is that the greatest treasure, "One Piece" left behind by the legendary pirate, Gol D. Roger, is said to be on the final island of Grand Line ("Raftel"). The Grand Line is divided into two halves by the red line. The first half, known as "Paradise" to the residents of the second half of the Grand Line, is where most of the action has taken place, and where Luffy has found half his crewmates so far. The next half, which is said to be much more difficult and dangerous than the first half, is the "New World". The Yonkou unofficially rule the New World.
Calm Belt
The Calm Belt is a sea area that surrounds each side of the Grand Line. No winds or sea currents can be found there, hence the name "Calm Belt". As a result, sailing through it is extremely difficult, especially because it is the nesting ground for the Sea Kings. However, Rayleigh swam from in the Grand Line to Amazon Lily in the Calm Belt by himself after his ship was sunk in a storm.Red Line
The Red Line is the one and only continent in the One Piece world. It traverses the entire world from north to south. The Red Line separates the two oceans into four smaller seas. One of the oceans forms the two seas, East Blue and South Blue, and the other one forms the North Blue and the West Blue. The Red Line is used by those traversing the Grand Line to separate the first half of the Grand Line and the "New World", as well as the initial starting place of one's journey through the Grand Line; it also exists as the only way to enter the Grand Line besides traveling over the Calm Belt, a highly dangerous prospect. The Holy Land of Mariejois is located on the Red Line, at the part where the Grand Line meets the "New World"Sky Ocean
The Sky Ocean is a place that is unknown to most people in the Blue Sea. The reason for that is that the Sky Ocean is entirely made of clouds, which from the Blue Sea doesn't look like an ocean. The Sky Ocean clouds, however, are no normal clouds. There are two kinds of clouds, Sea Clouds and Island Clouds. Sea Clouds are just as water, which means that you can sail on it if you´re not to heavy too sink through it, whereas Island Clouds are dense enough to carry all kinds of things.
There are different ways to get to the Sky Ocean from the Blue Sea. The only way that has been shown so far is the unorthodox and dangerous way in which the sailors have to take a Knock Up Stream, though it has been hinted that there is a second, slightly less dangerous way. In the Sky Ocean there are two known seas, the White Sea and the White White Sea.
