Wide angle lenses are the staple for underwater production, but why? Do you really need one?
As we go underwater the natural light diminishes, even on shallow sites. Using external lighting, video lights or strobes, drastically improves images. It helps add in lost colors, provides proper contrast, and captures the real beauty of the critters. For still photography, strobes are preferred. They provide lots of light in an instant, more than video lights can provide, and have other features, like TTL, to adjust how the camera and flash work together.
You can see how the image on the right is capturing more at the same distance from the lens. Both images were taken with a Sea & Sea DX-6G
The problem with lighting is that it can only be so bright, and only go so far. Once you get in close enough for the lights to be effective you're stuck with a closeup shot that may not have everything you'd like in frame. This is where wide angle lenses come into play. A wide angle allows you to use external lighting, get close enough for the lighting to be effective, and have more in your frame.
Some camera systems only have one lens, and that's what's used in all situations. More experienced photographers who want the most of their time underwater will use a dual system to capture different kinds of images on one dive. This is typically accomplished with a "wet-lens". A wet lens is a wie angle lens that can be attached to the camera during a dive. The Sea & Sea DX-6G wide angle is a perfect example of this.