Topography is the relief in an area, or the pattern of elevation of different features in a region. Let's talk about seafloor topography which is the shape of the seafloor. Although submersibles are definitely the most fun way to do this type of research, they are not practical for mapping the whole seafloor.
Submersibles can only cover very short distances underwater and can only dive to certain depths.
Another way is by using sound waves. Ships can emit sound waves as they are traveling along their route. These sound waves travel downwards until they hit something solid enough to reflect off, such as the seafloor. Once the sound waves hit the seafloor they are reflected back to the ship. These methods are much more rapid than mapping the seafloor by submersible, however the information received using these methods only reveals a narrow area underneath the ship.