Hi Abtaucherli....
The FCON-T01 is screwed directly onto the camera using the adapter ring (45 mm thread).
For the underwater housing you need the PTWC-01 underwater wide-angle converter (67 mm thread).
Depending on the housing model, an additional adapter is required.
PT-050, PT-054, PT-EP05L and PT-EP06L: direct attachment
PT-048, PT-051, PT-052 and PT-053: Use with step-up ring PSUR-03
Personally, I would invest the money for lens/converter in an external flash for now. (If you dive so deep that you need a UW housing, light is more important than perspective).
Greetings & good air !