Due to the curved lens of the waterproof housing for the HD HERO and HD HERO2 cameras and standard def Wide HERO cameras, you will notice a slight loss of sharpness/focus to underwater images.  In video mode with the standard def camera this is not very noticeable due to the lower resolution.  With the HD HERO and HD HERO2 cameras this becomes more evident at the higher resolution.  The Dive Housing has a flat glass lens which allows for the same image sharpness that you enjoy with your GoPro camera above water.  This housing is no longer available from GoPro directly, but you still may be able to find it at local retailers or online websites.