Sea Currents

Sea currents (海流) are forces of major importance in the world of One Piece. Since the world is predominantly covered with water, currents are used to propel ships to certain destinations at greater speed. Currents are usually created by changes of temperature in the water, and difference in salinity (the concentration of salt) levels, though there are some exceptions. The only place where you can't find sea currents are in the Calm Belts. It is said that all currents eventually connect to each other, rising up and down like a dragon. The Grand Line itself is thought to be one enormous current that seperates the seas.

Surface Currents
Surface currents are currents found in the warmer waters closer to the surface, and are quite fast moving.

Deep Currents
Deep currents are currents found in the cooler waters closer to the sea bed, and are quite slow moving. It is said that if a ship enters the wrong deep current, it can be trapped in the current for over two thousand years before it can reach the sunlight.

Knock Up Stream
It is considered as a natural disaster by most, but Nami describes it as a powerful current going straight up. It is created when water gets trapped in an underwater cave, where it is super-heated by lava beneath it, until the pressure gets too much, causing the sea to explode upwards. The torrent may vary from time to time, but at once it was powerful enough to send a portion of an island up into the Sky Island.

Tarai Current
The Tarai current, or the tub current, is a massive whirlpool that connects the three major World Government facilities. Its flow is controlled by the three Gates of Justice, and will either flow or cease based on how much of any of the three gates are opened. The current goes in a circle within the three gates, and is faster for ships to travel using this current than going around the perimeters of the gates to reach any of the three Government facilities. However, this only works if the Government is willing to open the gates for the said passengers.

Serpent Currents
On the way to Fishman Island, somewhere between the Florian Triangle and the Red Line, there are a series of currents that tend to leap out of the water. In the anime, the Straw Hats had to literally battle through them before they could proceed any furthur.

Downward Plume
Described as a massive "underwater waterfall", it is located near a winter island that makes up part of the Red Line. It is used to get to Fishman Island more quickly.

Reverse Mountain
Reverse Mountain is a place where currents of the 4 Blue seas converge and flows into the Grand Line. The currents are extremely powerful as they are able to pull ships up a steep slope.