Externet wrote:

What does a plain standard and normal registered boat, if modified to
very unusual characteristics has to comply with in order to legally run ?
Does it have to be presented to some authority for evaluation ?

That depends very much where in the world you are. Most likely the
authorities are more interested in collecting tax than in seeing how well
it works. They may require an insurance, and the insurance company has
reason to be more interested in the safety of the thing.

Of course, if you start making money with it (carrying passangers, cargo,
fishing, even selling it) the tax authorities are even more interested, and
some other authority may even care about the safety of it all.

-H