Many underwater photographers prefer to use very wide angle lenses.  A wide angle lense allowes them to get closer to their subject.  The closer you can get to your subject underwater, the less water is between the camera and what you are trying to photograph.  This means less color shift, better lighting, and less particulate to interfere with the quality of the picture.  Wide angle lenses also have a deeper focal range, so your pictures will more likely be in focus.

Distortion in pictures taken with a wide angle lense occurs at the edges.  Straight lines appear to curve around the center.  Lines that pass through the center of the picture remain straight.  This is because the picture that the lense takes is focused at a spherical surface, like the retina of your eye, but the sensing device in the camera is flat.  The wider the lense the more apparent is the distortion.  The best way to handle this type of distortion is to keep straight lines out of the picture or make sure that they pass through the center.  Fortunately, most underwater pictures are of natural subjects which have few, if any, straight lines.