Sea Frogs 67mm Wet Dome Port
When underwater and using a flat port (where the lens meets the water) this will cause a loss of angle of around 28%. Using this dome port will restore the field of view, and it also enables those great shots which are half in/half out of the water.
Note that this is a wet port, so the space between the front of the housing and back of a wide-angle lens must be filled with water in order to achieve the desired wide-angle effect.