We see this happen all the time here in Hawaii: Tourists go snorkeling — sometimes for the first time in their lives — and they are excited by what they see. They decide they want to take pictures of all the pretty fishies and corals to show their friends back home. They buy a single use waterproof camera, they snap away, and they are sorely disappointed when they see the result. Most of the photos are blurry, and though they thought they were shooting in color, all of the images are monochrome — blue monochrome.
For quite a few reasons, taking photos underwater is very different from taking photos on land. For one thing, you have to shoot through water which, as you know, is a much denser medium than air. The farther away you are from the subject, the less likely it is that you will get a clear, crisp image, regardless of what kind of lens you select.
The main difference, though, has to do with light. As sunlight penetrates water, it is gradually absorbed. The deeper you go, the darker it gets. The longer wavelengths — the reds and yellows — are absorbed faster than the shorter wavelengths — the blues and greens. Things look bluer the deeper you dive, because the longer wavelengths are effectively gone.
Now, there are many instances in which shooting photos underwater in natural light can produce effective images, but even at very shallow depths in very clear water the result will be quite monochromatic. You can’t fight with the laws of physics.
In most cases, lighting a photo subject artificially with strobes will be necessary in order to capture the colors and details. Here are some examples of sea turtle photos that will illustrate the above points.
This first image happened by mistake; my camera strobes failed to fire. This is a typical natural light photo at relatively shallow depth – the blue monochrome effect. Pictured is a rather elderly Green Sea Turtle (Chelonia mydas) sporting a Remora (Remora sp.) — A.K.A. ‘shark-sucker’ — on its carapace, photographed at Pulau Sipadan, Malaysia.
The second image also was shot in natural light, but just below the surface and at very close range. This Green Sea Turtle passed right beside me just as I began my descent from the surface. I had no time to do anything but aim and shoot, however since the turtle was so close to me and we were barely two meters deep, the details were captured reasonably well (including that gunk on the carapace). Notice, though, that the background is quite blurry and blue. This image was shot in Hawaii.
In the final example, another Green Sea Turtle at Pulau Sipadan was lit with a single strobe from the right side of the frame. The markings on the turtle are well captured, and the soft and hard corals in the foreground are well defined. The fishes in the background are fuzzy, which is typical for this kind of shot.