Why a seastead still needs registration and port access

Could an unregistered cruise ship conduct cybercrime without answering to a state?

No. An unregistered vessel invites boarding, and choosing a flag still brings legal obligations.

An unregistered ship would invite boarding and inspection, and removing a vessel from one register without joining another is described as difficult. Registration brings the laws of the chosen flag state with it. Ships also need fuel and maintenance, so remaining afloat does not remove their dependence on ports. The Satoshi project provides the example: Panama registration still imposed obligations, while the recollection describes other states refusing access.