Another reason for using a proper underwater camera is lens aperture. The iPhone camera has a tiny aperture, which lets in a limited amount of light, causing noticeably darkened photos quite near the surface. The flash on the iPhone wouldn't be useful underwater as it's too close to the lens, causing every particle in the water to reflect the light from the flash right into the lens, where those reflections will be very annoying in every photo. That's why underwater cameras have the flash mounted quite far from the lens.