Forever Fleet Nation (FFN) can declare its territory anywhere no other nation has claimed influence - which is basically only possible outside the 200 nautical mile Exclusive Economic Zone of other countries. This means that areas such as the Baltic Sea, the Mediterranean Sea and most part of the North Atlantic are off-limits or impossible to get into and out of (except by permission from controlling nations, as ambassador or temporary visitor).
Much of the rest of the world's seas, especially the oceans, are however wide open for Forever Fleet Nation to declare territory by having a permanent presence, usually by a FFN vessel permanently anchored from which the FFN Exclusive Economic Zone is projected.
Below is an illustration of the world-wide 200 nautical mile Exclusive Economic Zones (and fishing zones) already in existence. Anything white is where FFN can claim territory.