When taking wide angle photos underwater we observe a remarkable effect: if we look up in quiet water, it appears that we can look at the world above water through a circular window.
Circular window
The reason is found in Snell’s law, which states that light is refracted when going from one medium to another medium with different ‘refraction index’. In our case light goes from air to water. Light propagates not in a straigt line, but is refracted.
Flash, Forrest, flash!
Many starting photographers conclude there is not much light underwater and start flashing. Many using one flash, more luxury divers use two flashes to prevent shadows. Lazy photographers close the diaphragm and blast out a hell of flash light, having always sharp photos due to the large depth of field and not having to worry about white balance. This however, has some drawbacks:
Flashing casts shadows
Backgrounds appear dark
Light comes natural
Photographing using natural light has advantages, when done properly:
Naturally illuminated pike