I'd expect it's some combination of "can sharpie a rock and immediately sell it for a hundred thousand dollars" and rich patrons who are willing to fund things like this to say they did a project with Banksy.

Selling goods and services for money?

Plus, it looks like a relatively humble boat, compared to the numerous superyachs you see a few hours sailing away in Cannes / Nice / St Tropez for instance

Their art sells for millions and they have several published books.

hah, his works are worth millions.

edit: He should just buy 10 more boats and ignore the bureaucrats. People would probably give him money to do it. (I would)

So a warlord outside the law. I think at some point the italian marina should sink the boats

And I agree about ignoring bureaucrats and straight away deporting all immigrants on the spot