@DocTock: thanks a lot for the advice! Which port are you using or is the Nikonos used as a port? But how is then the connection to the camera? Do you have a photo how this looks like? Lots of questions but thank you for every piece of information!!!
To the OP @oneyellowtang - the advantage for me to the RS conversion is it allows me to carry a full macro and WA kit in my photo-backpack.
The Nikonos RS lens is a port with a lens.
In the image below, you can see there is a dark metal ring between the Nauticam housing and the RS lens (dome portion).
That is the adapter (has o-ring on inside) that is needed for this Nauticam - RS solution.
To attach the lens, you first mount the adapter to the housing and the camera to the housing.
Then you attach the RS unit (it has it's own o-ring) to both the housing and the camera body at the same time.