Pentax made their own marine housing for the 6x7 including a porro-prism, but it was a special order item and consequently quite rare. I have seen various parts of the system on eBay but they are very expensive. Apparently Cameratech in San Francisco bought out the remaining parts from Pentax.
A scan of the original manual is available on the Pentax USA site:
Look for "6x7 Marine" in accessories.