Water presents some special obstacles for the photographer. Being
much denser than air, water supports more things. Organisms, silt
particles, bubbles and the like all stay suspended in the water between
the lens and the subject, blocking light, reducing resolution, and
lighting up like Christmas lights when you fire the flash. In
addition, the water itself absorbs light, and does so in a selective
way. Reds and yellows drop out quickly, and light passing through
water soon takes on a bluish hue.
There are other practical concerns. Water is
tough for most of us to breathe, and it's hard to use a tripod hanging
suspended 10 feet above the bottom. On the other hand, the density
of water reduces camera shake and it's easy to get over obstacles or
hover in just the right position. Clearly, underwater photography
is a different ball game.