Updated: Oct 15, 2019
This is a big subject and there are several lines of defense for Forever Fleet Nation:
It is not unusual for nations not to have a military; this is true for 32 nations in the world as of 2018, and the number is slowly rising it seems.
Many of these nations do have a part of the police force taking the role of border guards and similar, but with Forever Fleet Nation having no central police and no borders (except for directly entering the FF vessels), this is not meaningful for FFN.
As such, Forever Fleet Nation instead counts on many other forms of defense.
The first few demonstrator vessels are far within other nations' security zones, as are the embassy vessels. These vessels will need and have none or little security personnel, since they are under the protection by the police and/or military of the host country.
The first prototype vessels and any FF vessels outside the 200 mile Exclusive Economic Zone of other nations are generally so far removed from other nations that they are under little threat from anything but the most committed pirates or a nation seeking war.
The option to join
Much piracy and small-scale nation conquest is based on desperate resource deprivation; an issue Forever Fleet is designed to deal with either by providing help or letting the stricken join Forever Fleet Nation.
Alone the existence of Forever Fleet Nation will as such reduce the risk of piracy and conquest since regions in crisis near the sea will get much needed help from the Forever Fleet vessels long before the crisis devolves into the desperation that breeds piracy.
And in the cases where Forever Fleet for some reason is not helping, the prospects for would-be pirates with access to a ship that can reach FF vessels 200 miles out in open seas are much better if they join Forever Fleet and receive upgrades to make their own vessels sustainable for themselves and their loved ones, rather than try to take over an FF vessel.
Individual vessel self-defense equipment
Most FF vessels on the high seas will be equipped with anti-piracy equipment, and their sheer size and power surplus will make them well-equipped for using water, sound and laser based non-lethal deterrence weapons, with some of these often also acting as anti-weapons capable of neutralizing incoming grenades or missiles. Such anti-weapon equipment is not unusual on civilian ships, with even some newer yachts rumored to be equipped with military-grade anti-air systems.
Individual vessel security personnel
Most FF vessels, both on the high seas and when acting as embassies inside other nations, will have a few security trained people among its citizens taking on the roles of guards of the vessel, as well as perhaps acting as police in situations on board where this is required by the other citizens.
These security personnel will usually also have training in small arms use, as well as the non-lethal deterrent weaponry and anti-weapon equipment. A few warning shots from a high caliber rifle, from a trained shooter well protected on the FF vessel, will deter most pirates from even thinking about approaching closer, even if they have braved the non-lethal deterrents.
State alliances and corporate alliances Forever Fleet will have mutually positive ties to many states, corporations and international organizations, who will not look favorably on anyone attacking Forever Fleet.
As such any state, corporation or group attacking Forever Fleet in any way, will quickly find itself in trouble from many directions, facing international opposition, boycotts and embargo which will quickly make any stupid dreams of the systematic capture of FF vessels or citizens more expensive than any possible gains.
United Nations intervention
In case of major threats to Forever Fleet Nation, when officially recognized by the United Nations, a plea for help can be made to the UN while the special scattered makeup of Forever Fleet Nation will stall any attacker for a very long time, yielding only a few vessels. This will take a long time for the UN to decide, but eventually require the UN to step in and force the aggressor to back out, either through massive sanctions from the UN members combined, through the threat of UN military action or if need be through said military action against the attacker.
The Cyber Defense deterrent
With the massive computer power available to most FF vessels and the competence in cyber security and warfare held even just by some of the citizens already having joined Forever Fleet, making cyber attacks against Forever Fleet Nation will be almost impossible, and in any case prohibitively expensive compared to the gains.
This cyber warfare capacity can however also be used offensively in a desperate situation where all other more civil options have been tried and failed, or while the UN ponders the options for intervention - with decentralized but coordinated cyber attacks from a multitude of FF vessels around the globe harassing or completely debilitating an attacker, be it a corporation, state or group, the attacker must be extremely persistent not to withdraw simply to cut losses: Remember that today cyber attacks can completely paralyze an attacker or a whole nation, even physically ruin national infrastructure and military equipment, as well as embarrass corporations and individuals entirely out of their power.
The centralized FFN Special Forces MAD doctrine
Where everything else fails, Forever Fleet Nation has a last gruesome defense to unleash, well known from guerilla warfare; sending the highly trained FF Special Forces otherwise guarding the most valuable infrastructure, to hunt down and capture or execute the most high-ranking person(s) behind the attacks on Forever Fleet Nation.
This should ever only be conducted with the sanction of United Nations or similar humanitarian body, to never become a tool used offensively and always be a last ditch effort only kept as a way to hopefully deterring conflicts from ever escalating to this level.
Still even if never used it makes a very efficient MAD (Mutually Assured Destruction) doctrine akin to nuclear warfare, since no leader ordering an attack on Forever Fleet Nation can then ever feel safe, and the cost of doing so will almost invariably be deemed too great. This seems to be the most effective way to defend a nation today, especially a globally scattered nation such as Forever Fleet in a future where the existence of UN is uncertain. It is however a MAD doctrine, since if Forever Fleet Nation makes use of it, its leaders cannot expect any different retaliation so literally risk their own lives by making use of it.