So, you've got it. It arrived in the post or you picked it up from the dive shop. Maybe another diver gave you a deal when they upgraded. No matter how you slice it though, you've got an underwater camera and you can't wait to try it out! You've been telling your non-diving buddies and family all about all the awesome stuff you've seen, and now you can show them! Here's a few quick tips for the first time underwater photographer to help you get over some of the initial humps of figuring out how to take pictures underwater.
Figure out your camera before you hit the water. Put it together, play with it, figure out what buttons do what and get used to handling it. Test out the housing in the bathtub or in a pool to make sure it works. It may sound weird, but all this gets you used to operating the camera and more comfortable with what is probably the most complex item you'll be operating on your dive.
Buoyancy and control are your best friends when you start taking pictures. Personally, my buoyancy was iffy at best until I had to get a grip on it to take better pictures. Being still and nonthreatening is key to taking picture of marine life, especially fish and mammals! Steady breathing and steady hands help make good pictures.
Remember that shades of visible light are filtered out the deeper you go. Only blue is eternal in the sea, with all other colours fading out one by one as you descend. Use your flash and flash diffuser, and if your camera has underwater settings, play around with them. Don't hesitate to get close to your subject if it's safe to do so, and remember that your camera's range is greatly reduced, so panoramic shots in anything but shallow water tend to be a little... washed out.
The biggest thing though, is be ready for frustration. Pictures will be out of focus. Fish will move suddenly as you finally get them lined up. The current will be too fast our you'll have too much momentum and overshoot your target, getting a blurred photo as a result. This is normal. It's part of the learning process, and even if you've taken thousands of pics, it still happens. If you get one pic in 10 that's good, you're on a hot streak. And when you take your first really good picture, you'll never forget it, or the feeling.