HERO GEAR sells a range of Filters for GoPro Cameras

Red Filters:

Red Filters are used for Scuba Diving in Tropical or Blue water. Below 10 feet of water, the wavelength of red light disappears. The camera white balance does not account for the loss of red light and videos turn very blue or green, lacking bright tropical colours. This red filter tricks the cameras 'brain' into capturing vibrant colours the way you see them underwater!

Snorkel Filters:

Snorkel Filters act in a similar way to Red Filters but are less Red, due to the fact that most snorkelling is done in shallow water.

Magenta Filters:

Magenta Filters are designed for colour correction while scuba diving in green water.

Polarisation Filters:

Polarizer filters help reduce glare. When filming outdoors sunlight reflects off of many different objects: For Example Snow, Water, Roads, Trees, pretty much everything. Polariser Filters reduces light bouncing off these objects allowing the camera to capture more detailed and colourful images.

Neutral Density Filters:

While filming in bright conditions the ND filter will reduce excess bright light and reduce overexposure and blown-out video. 

The Filter will also reduce the shutter speed creating a blur motion. It also fixes the common "Jello Effect" or wobble commonly associated with RC Planes/quadcopters, as well as Motorsports.

Brands include: Polar Pro and GoScope.