Since a seastead is both a small island nation and a boat, we must consider threats that apply to both.
For the threats that apply to an island nation, the most recent historical examples are Great Britain and Japan during World War II.
A successful invasion would allow the attacker to have complete control over the seastead platform. Possible motivations include:
- desire to take over the platform and operate it oneself
- denying the platform to its current owners
- terminating some of the operations of the current owners (e.g., tax haven)
- arresting and repatriating some of the platform residents
Invasion by direct amphibious assault is likely to be costly to the attacker. Unfortunately this is likely to make an attacker consider just sinking the platform instead.
It is unlikely that the seastead could resist a determined invasion attempt. In a way, if an invasion force ever assembles, the seastead has already lost. The goal should be to prevent an invasion by making an attack on the seastead politically unacceptable to anyone with the ability to launch one.
A far more serious risk than invasion is blockade. Both Great Britain and Japan were severely harmed by blockade during World War II, and trends in shipping -- fewer larger ships used to move more cargo -- since than have made blockading an even more effective strategy.
A hostile force could create a Wikipedia:Total_Exclusion_Zone total exclusion zone around the platform, threatening to attack any ship or aircraft that entered it. If the blockading force patrolled the perimeter of the zone, 200 nautical miles away from the platform, mounting an attack against the blockading force would effectively require the seastead to have a 'traditional' navy.
Another possibility available to a blockading attacker would be to simply announce that any ships that call or plan to call at the seastead are subject to attack anywhere in international waters. Since the seastead would have a relatively small flow of regular shipping (one or two supply ships per month), it would be easy to identify the ships that regularly call and then sink them at the convenience of the attacker (e.g., when they are close to the attacker's territorial waters/navy).
Defense by Self-Sufficiency
Self-sufficiency in all necessities makes blockade a less damaging strategy, and self-sufficiency should be explored for that and other reasons. But a blockade would still disrupt the arrival of luxury goods, mail, and new residents.
A seastead could be sunk by concerted attack. As with Invasion, above, this is the sort of attack that is only likely to be available to modern large navies using either guns, anti-ship missiles, aircraft-dropped bombs, or small nuclear warheads. As with invasion, the seastead's defense against this sort of attack should be based on public relations.
- pirates are unlikely to be interested in attacking seastead
- flag-of-convenience seastead cannot rely on flagger's navy to fight pirates
- no-flag seastead cannot rely on anyone to fight pirates
- risk is not merely direct attack on the seastead but also attack on supply ships
Example of this threat: A US Coast Guard ship steams up to the seastead and demands that a particular person be transferred from the seastead to their custody. The "or else" is left vague.