Being able to display oceanic properties on a map is an essential prerequisite in oceanography. Maps are two-dimensional representations of the surface of the earth. We know that the earth is (nearly) a sphere, so we have to find a way to project a spherical surface onto a plane.
How this is done is the essence of the science of cartography. In the context of oceanography we restrict ourselves to a very brief review of the essentials.
The essential elements to consider in a projection are:
Area: Fidelity of area is essential if the map is used to compare different parts of the earth's surface with each other.
Angle: Fidelity of angle is paramount in maps used for navigation, which is based on compass bearings, i.e. the angle of the course of a ship or aircraft with respect to the magnetic north pole.
Orientation: The preferred map orientation is that, for all points on the map, north is up and east is right.
Ease of matching: It should be possible to combine a map of (for example) the Atlantic Ocean with a map of the Indian Ocean without having to redraw the entire map.
Cartographers teach us that it is not possible to combine all these essentials in a single projection. Different projections exist to serve different purposes. The correct choice of map is an important part of displaying information in oceanography.