Span the entire globe, can be in Brightwaters to mid-Gloomwaters, and is how many travel quickly from place to place. On maps generally marked by semi-translucent lines with arrows to show the direction of the flow. Those marked on maps are considered 'stable', meaning there maybe a variance of location within a reasonable distance and a variance of speed depending on the time of year and general weather conditions, but will be there and findable without too much difficulty. Smaller currents may be known locally, but are less reliable and rarely marked on maps.
Photo by Pixabay: https://www.pexels.com/photo/macro-photography-of-water-waves-355288/