Delaware registered boats must be registered in Delaware as its “State of Principal Use”. Delaware regulations state that a vessel must be used, docked, or stowed on the waters of this state for over 60 consecutive days per calendar year, to count as “Principal Use”.
A vessel registered by the State of Delaware does not obtain USA nationality under UNCLOS. Accordingly it will be assimilated as a vessel without nationality and will not receive the rights, obligations and benefits associated with such nationality. A vessel is not flagged by any US State Government, flagging is only provided by the United State's Federal Government.
Poland is a nation state. The vessel never needs to be in Polish waters for a minimum period and never needs to visit Poland. Non-Polish citizens can register their boats in Poland; Any nationality owner can register.