Neorealists typically suggest that when it comes to matters of international politics, non-state actors (NSAs) count only in so far as they act as agents of nation-states, and that if they are not such agents, then their role is merely epiphenomenal, so essentially irrelevant. This thesis argues that this is certainly not true when it comes to key areas of concern for international maritime security. NSAs here may employ the traditional tools of advocating and lobbying that leave neorealists unimpressed, but they do more than this. Increasingly they have forced policy changes by assuming roles that states cannot or will not perform at sea. In making this argument, this thesis draws on three different case studies; irregular maritime migration, piracy, and illegal, unreported and unregulated fishing. In each of these cases we find that the actions of NSAs at sea have a direct and contributory effect on the way states are or are not responding to the security concerns in international waters. This capacity is clearly bound up with the absence of sovereign control over the high seas. The long-standing 'freedom of the seas' doctrine opens a space for conflicts between the state-centric conceptions of security and national interests of states on the one hand, and (human) security, trade and environmental concerns on the other, that NSAs are well placed to fill.