Flat or Dome Lens Ports
You learned in your open water course that objects underwater appear about 25% larger than they do in air.
The flat port is unable to correct for refraction. Dome ports also known as Wide-Angle ports are used to correct for the refractive properties of water.
Light is refracted 25 percent causing the focal length of your lens to increase by 25 percent. The dome port is a concentric lens that reduces the problems of refraction. All the rays of light pass through unrefracted which allows the lens to retain its angle of view.
Below are examples of scenes with the same lens and different ports.
Flat ports, also known as Macro ports
For macro photography you gain greater magnification of your subject. For a close-up with a long lens, the flat port is a better choice. Macro can be shot behind a dome port image quality will actually be better than behind a flat port, because of less chromatic aberration, and because you must get closer resulting in less water between you and the subject. Magnification will be 30% less when using a dome port.
If you use your wide-angle lens in a macro port, the photo will vignette. Wider lenses can also suffer from chromatic abberation.
Port extensions extends the port for the longer lens. Most wide-angle lenses will require an extension with the dome port so the lens will fit inside the port and also wide angle lenses must have proper placement inside a dome for optimal sharpness.