The glass flat lens (patent pending USPTO 29387406) available from Pursuit http://freedivingspearfishingg…ens-for-gopro-camera.html fixes the underwater focus/blurry problem caused by the original Gopro curved lens. With the flat lens the GoPro camera can take crisp high definition undewater video. The flat lens is a drop-in replacement for the original GoPro housing curved lens. It has a depth rating of up to 180ft, the same as the Gopro housing. The lens is made of glass set in an anodized aluminum frame.
Spearfishing video shot in high definition with the GoPro Hero HD and a flat lens. Courtesy of greekdiver.
http://www.youtube.com/watch?v=Kr_JwvSxlsM&feature=player_embedded
Video demonstrating effect of the flat underwater lens at the 5 different available settings; R1, R2, R3, R4, and R5. R5 is the most suitable high definition setting for filming underwater and has no vignette effect.
http://www.youtube.com/watch?v=t6gOEIYBfog&feature=player_embedded
To install the flat lens undo the screws of the black plastic bezel with a precision Phillips screwdriver, remove the bezel and set it aside leaving the screws in it. Take out the curved lens, there's a black gasket that goes between the lens and housing that is usually stuck to the lens and will come out with it. Separate the gasket from the lens and place it back in the housing. Put the flat lens on top of the gasket, no sealant or glue is needed. There's only one way in which the lens can be positioned for the bezel to seat back and the screws align with the screw holes, so there's no worry of assembling incorrectly. When tightening the bezel screws do it in a crisscross pattern to tighten it down evenly. Do not over tighten the bezel screws. Over tightening may compress the rubber gasket too much, making it expand into the field of view of the camera.
Housing and lens disassembled, with the glass flat lens on top of the housing.
I often get asked if it's difficult to install the flat lens in the Gopro housing. What better way to demonstrate how easy it is than by doing it outdoors on a breezy day, on a small uneven work surface, and installing two lenses at once. Adding the risk factor of dropping something in the water for extra tension 
Total install time less than 5 minutes. Video was shot with a Gopro in R5 setting, mounted on a head strap.
http://www.youtube.com/watch?v=VLODlogMQQY&feature=player_embedded