This set of tips might help. The first 2 are pretty important but from there down it's the order they came to mind. The sample I'm including shows that post processing after taking the shot is important & can make a big difference to your photos. If I decide to print a photo I spend much more time on the post processing than when it's going into an on line album at reduced resolution. I use Photoshop to process my RAW images but I use the free program Google had on line but recently abandoned called Picasa. It's still available but there won't be any future updates.
Buoyancy control is the number one thing needed for good photos. With is set correctly your hands are free to operate the camera.
Distance to subject has a major influence of results. The more water there is between you & the subject the more silt & debris there is to ruin the photo. That water also filters out the light from above & from your flash (strobe) but the flash makes the waterborne debris look like you are shooting through snow flakes.
You should know & understand your camera’s controls so that you can change the exposure the camera sets based on it’s built in light meter.
Basic underwater housings usually come with a simple light diffuser for their built in flash but that also cuts the power of the flash reducing it’s effectiveness. External strobe(s) are a very important part of underwater photography but they are also expensive. Because most basic set ups rely on the built in flash to trigger added on strobes the camera has no way to know how powerful they are or to control output power levels if that is adjustable. One more reason you need to understand the camera controls & how to over ride them for a better exposure.
DON’T rely on using the AUTO setting of your camera. Program mode is similar but allows you to over ride the program’s decisions re exposure by letting you dial in EXPOSURE COMPENSATION, which allows you to increase or decrease how bright the photo will be. It takes a bit of practice to understand just how much to change it but it is essential to learn this.
MANUAL mode allows you total control of the settings & again the more you use it the better you’ll get at picking a good starting point.
Learn how to use the ISO settings to get a higher shutter speed when needed. A higher the ISO allows a faster shutter speed but at the expens
e of noise (static) in the photo. Because both you & most likely the subject are moving the shutter speed needs to be fast to stop the motion. Blurred photos are more often than not the result of camera shake & / or subject movement while the shutter is open.
You may not understand this so I’ll include a link to a nice & simple Canon Tutorial which is trying to explain the basic rules ALL brands of cameras follow to get a decent photo. It’s land based but since most are using a Point & Shoot camera in an underwater housing the important thing here is to understand Depth of Field (DoF). The smaller the sensor the greater the DoF at any given Aperture setting & most P & S cameras have sufficient DoF wide open for underwater photography & when used in Auto or Program mode the camera is most likely going to choose the widest (brightest) aperture available which then lets the built in program use a fast (for the available light) shutter speed. Increasing the ISO will also allow a faster shutter speed but remember it also adds noise above a certain level. ISO 200 will be clean compared to ISO 800 or above but it may get a shot that would have motion blur at a slower shutter speed.
Most of the subject matter you want to shoot is afraid of you. Be smooth & gentle as you approach it with your camera ready to use.
Because every camera is a bit different & the more expensive it is the more features it may have so this relates to the features that usually help if built into the camera. Set the AF to SERVO mode, which tracks moving targets while the shutter button is half pressed. This works whether it’s the current moving you or the fish is in motion. Use any built in Stabilizer to reduce the effects of camera shake. Shoot in RAW & correct the white balance after the fact when processing your photos OR if you don’t have RAW available learn how to set a custom white balance & re set it as the depth & light changes. I’m going to pick one photo from my recent trip which most people would be happy with as shot but I will do a 1 click White Balance correction to the RAW image to show how important that White Balance is. I shoot RAW plus jpg but many will just shoot RAW all the time.
Tutorial
Get the Perfect Shot with Canon