For those who use a DSLR for underwater photography, I have enclosed a list of recommended lenses that should cover the spectrum of photographic opportunities.

U.N. Wide Angle

Made by the same company that developed the housing line for Olympus. The U.N. lens features glass optics and yields a very sharp image. Coverage is around 100 degrees, depending on your camera. Depth of field is enormous so it ís easy to shoot close focus wide angle images or large subjects like whale sharks and have it all in focus.

Inon Mini Wide

The Mini Wide is an ultra compact wide angle lens. It provides around 85 degrees coverage - roughly the same as a Nikonos 20mm lens. It is a razor sharp lens and focuses down to a few centimeters with great depth of field for crisp images from near to far.
The Mini Wide uses a bayonet fitting. Adapters are available for some Olympus and Canon housings. The Mini Wide is not suited to larger housings designed to use 67mm fit accessories as there will be image cut off. The bayonet mount allows almost instant lens changes with just a 45 degree twist of the Mini Wide.

 Inon Super Wide

The Inon Superwide is an extremely interesting lens for the serious digital photographer. The primary lens uses glass elements and offers a field of view of around 100 degrees (similar to a Nikonos 15 mm lens).
Image quality is excellent, as you would expect from Inon.
What sets the Super Wide apart is its ability to accept a dome port.
Adding the dedicated dome expands the field of view by around 30 percent. This makes the Super Wide especially attractive to those working in low visibility, wreck photographers and divers shooting large animals like Basking or Whale Sharks.
The dome port is constructed from optical glass. And, because it is by Inon, it ís even multi coated for better light transmission and to minimize reflections. A metal lens hood is incorporated into the design.
The Super Wide can be used with housings that have filter threads up to 67mm in diameter. This includes many Olympus and some Canon models.

Macro Lenses

Most digital cameras have good close up capability built in. For many small subjects this may be all you ever need. You can usually photograph large nudibranchs, brittle stars and cup corals so that they fill the frame without accessory lenses. Getting smaller creatures to fill the frame may be more difficult. Some cameras simply aren't equipped for extreme close up photography. With others there are lighting issues. At very short camera to subject distances the built in flashgun cannot properly illuminate the whole of the picture, leading to intrusive shadows. This problem is often exacerbated when the camera is inside a housing by the bulk of the hull. Some cameras cannot shoot extreme close ups or supermacro as is it sometimes called with their built in flashguns. So they automatically turn off the flashgun . This means that they also cannot trigger a slave flash mounted externally such as an Inon 180.
To solve these problems close up lenses can be mounted to the outside of some housings. These normally maintain or even increase the magnification of the cameras own close up lens. By maintaining a distance (typically 20cm) between the camera and your subject it is easier to light from corner to corner either with the cameras own flashgun ( with some camera/ housing/lens combinations the built in gun is obscured by the close up lens and you will need to use an external strobe) or an off camera strobe.

UN Close Up Lens

UN are well known manufacturers of underwater camera housings and are the company that builds the PT series of housings sold under the Olympus label.
The UN close up lens screws directly into any housing with a 67mm filter thread. For housings that use other sized threads adapters are included for use with 40.7, 46 and 48mm fittings.

Inon Close Up Lenses

Inon are renowned for their innovative flash systems and lenses. These close up lenses maintain Inons reputation for quality. The close up lenses are a doublet design. They use two lenses, separated by an air gap. The same principle is utilized in the Nikonos close up lens that is used by many professionals. Doublets are more expensive to manufacture than the simple single lens magnifying lenses supplied by most other companies, but yield a higher quality image.
Inon close up lenses are offered in a 67mm screw thread fitting or with Inons exclusive fast mounting bayonet connector.

Keeping Your Lenses Safe and Accessible

Having additional lenses available can make a huge difference to your underwater photography. Wet lenses are especially attractive as they let you switch equipment during the dive in exactly the same way a topside photographer switches lenses to suit different subjects, conditions and compositions.
Land photographers usually carry extra lenses in their camera bag or in cases slung over their shoulders or attached to their belt. These aren't practical solutions for divers. Even placing lenses inside BCD pockets leaves them vulnerable to damage and loss.
To keep your lenses immidately accessible and to ensure that they can be passed to you with your camera at the start of each dive and handed up and secured in a safe place at the end, lens caddies are usually your best option.
Lens caddies normally attach to your strobe arm and have identical fittings to your housing so that you can attach your lens to them for storage. Inon lens caddies are available with either a 67mm thread or their exclusive bayonet mount. You can choose from caddies for either one or two lenses.