Running a submarine is much more complex than a surface ship, even without considering the weapons. In a submarine, you cannot afford flooding or hydroplane failure. No wonder submariners are the elite in all navies that operate submarines. Almost all civilian submariners are ex-military.
I can't imagine doing this for pleasure.
Curiously, the phone
number on the web page is Austrian. Austria lost
its submarine fleet and its coast on the Mediterranean
in 1919... The design office "Starkad Technologies" appears to be Estonian with only 2500€ in assets: Starkad Technologies OÜ Estonia - Business Profiles