Getting the best blues and greens in the background is one of the most important aspects of taking successful underwater photos, and once you learn to work with those blue and green background tones you will find that you can do some pretty amazing things with color. By understanding how the color blue will look in your photos and by understanding what affects that color, you can greatly improve your underwater photography skills.
Strobes, Color Temperature, and Blue Color
To begin with, get to know your camera by taking plenty of shots topside in a dim or dark room. This will greatly increase your ability to work with the controls so you know what each function does and how it works before you ever find yourself at a dive site wishing you could get that camera to work. This relates to blues and greens in particular because in order to get those, you need to know how to tell your camera what to do. You can tell your camera what color temperature you want photos to be; when you choose higher color temperatures, reds and yellows will be added, and when you choose lower color temperatures, the camera's software will try to add more blues.
Next, remember that you'll usually have light from more than a single source. Sunlight creates great blues and brilliant greens, but metering off the water and playing with your exposure can help you to get blues and greens that are either lighter or darker than they appear to your eyes. Just spending an entire dive shooting nothing but water is an excellent way to learn how to manipulate your camera's settings to get consistent colors effortlessly in the future.
Foreground Colors Create Contrasts
While we're focusing on getting good blue and green water tones in your photos, it's important to remember that color contrasts in a photo can help to make various shades really pop. It's great to have foreground colors in your photos, so learn to use strobes and flashes to your advantage, and remember to get up close to your subjects to capture all the color you possibly can.
Silhouettes Can Enhance Blues
When you want blue tones to be the only colors in your photos, then try working with silhouettes. Getting the sun directly behind your subject is extremely important when doing this, so be sure to position yourself accordingly. On the flip side, shooting sunbursts and sunballs can create astounding blue tones, and can make even the most ordinary photo look amazing.
Strobes and Color
If your strobes are more than a couple of feet away from your subject, then you're going to lose a lot of warm colors like reds and oranges. If this happens, you can use your RAW editor to get better foreground subject color, but that takes some work. Try to capture colors right the first time and your results will be a whole lot better.
Use warm strobes for lower color temperatures to get the blues right; strobes close to 4500K tend to work well in most situations. Here's one place people shooting with compact cameras are at a disadvantage; strobes manufactured to get the most power from the smallest amount of wattage tend to be coolest. Use gels to warm them up.
Finally, it's important to get your exposure right to get the blues you want. If your shutter speed is too low, the result is overexposure and a washed out look. If they're underexposed, then the blues will be too dark. The trick is in knowing what your camera will give you at certain settings, so by all means, unless you want a really natural look like you'll normally get on clear, sunny days, play with the exposure to control those blues! You'll end up with interesting artistic effects. Finally, work with whatever conditions are giving you and be creative; you'll find that you can develop your own signature style this way, once you have learned the basics.