There are in fact many reasons why you would use this specialized equipment, but there are 2 main reasons large dome ports are used for creating over/under images. Magnification and Dome Radius. The curved surface of a dome port counteracts the magnification effect that happens to light when it passes through water. When viewed through a flat surface (like your snorkel mask or a fish tank) subjects underwater will appear bigger than they actually are. This is not a problem when your entire composition is underwater, but when you want to view a subject above and below the surface things begin to get weird... The large radius of a 16" dome also adds more distance between the camera lens and the water surface. This distance allows the lens to easily see above and below the water plane and also provides a smaller water line in your image which is less distracting and more visually pleasing.