The fact that you can keep a 5k camera dry underwater, does not necessarily mean that you can record 5k underwater images.
My tests (shot in controlled conditions and analysed with Imatest) show that shooting wide angle (90 degrees FOV terrestial lens) underwater results in approximately 17 pixels CA in the corner, which is approximately 150 lines per picture height. That's the resolving limit of a wide angle lens placed behind a flat port underwater.
Even underwater domes do not seem to resolve more than a few hundred lines in the corners.
3Deep is different. It uses Nikon Nikonos lenses specifically designed and corrected for underwater. They are submersible lenses and do not require ports (flat or dome) that cause great deal of resolution loss, distortions and chromatic aberrations. They resolve up to the Nyquist limit of Epic sensor corner-to-corner.
Can you tell us how did you manage to resolve 5k in the edges and corners of a flat port? To my knowledge this is not possible.