One of the major drawbacks of a compact camera is the size of the lens compared to DSLRs. The DSLRs get around this by the large array of lenses that are attachable to the camera from Super wide angle to Macro lenses allow the user to get sharp images. One of the key fundamental requirements of underwater photography is 'get closer' to the subject to remove excess water between the subject and lens. Compact cameras though are designed primarily for land shots, anything from one meter to infinite is the lenses typical sweet spot, not 10cm away.
To get around this various companies have developed wet lenses that attach to the outside of the camera housing. Many of these are interchangeable underwater which provides a major advantage over the DSLRs. After all you might be setting off to take photos of Seahorses with your macro lens but are rather stuck when a whale shark appears. Compact cameras with wet lenses means you can get both shots in the same dive... Wet lenses generally attach to a 'mount' which attaches to the tripod screw on the housing and rests on the lens port. There are some makes now that are providing the attachment (Especially M67) directly to the underwater housing without the need of a mount.
Generally there are three classes of wet lenses:
- Macro Lens - For all the small creatures and critters out there, magnifies the subject
- Wide Angle - All round lens for most subjects and most versatile, provides between 100 and 150 degree field of view
- Fisheye Lens - For very large features, provides up to 180 degree field of view
This site leans towards INON lenses and accessories, however other companies do provide similar support. I do though feel that INON offers the best all round for compacts and housings.
Below are two photos of a fan coral. The one on the left is without a lens the one of the right is with a wide angle lens from the same position:
The only issue with adding wet lenses is it will not be long before you start to add additional Underwater Lighting.