Current: A current drags creatures along its path. When you swim into a current, you move a distance and direction according to the current’s strength and in the direction where it flows. This is a slide effect, with the distance and direction determined by the current. If you wish to fight against a current, you can spend squares of movement to reduce the distance the current slides you. You can reduce the distance partially, or decrease it down to zero, provided that you have enough movement to do so.
If a current slides you into another square with a current, you ignore that square’s current. This applies to all squares the current moves you into, including the destination.
Current terrain on maps indicates the direction the current slides you and the distance in squares that the current moves you.
Other Terrain: Difficult terrain, cover, and concealment all exist in watery realms. The ruins of a sunken ship provide cover, while dirt kicked up by powerful currents grants concealment. Choppy, storm-churned seas act as difficult terrain. Best of all, underwater battles allow for up-and-down movement. Creatures can attack the characters from all directions, not just along the ground.
Published in Dungeon Master's Guide, page(s) 45.