The COVID-19 outbreak on international cruise ships during the early stages of the pandemic has exposed deficiencies in the governance of public health emergencies within the framework of existing international law. These deficiencies encompass various aspects, including the shortcomings of the system of flag state jurisdiction, the vague definition and reasonableness of governments’ “additional health measures” as stipulated in the International Health Regulations (IHR) of 2005, the role of World Health Organization (WHO) in the protection of the fundamental rights of passengers and crew members during epidemic outbreaks on cruise ships, the application of the free pratique rule under the international health law, and other challenges that have not been adequately addressed in current international law. In the post-COVID era, it is essential to revisit these core issues of international law and reassess the allocation of responsibilities among all evolving actors to foster effective multilateral cooperation in epidemic control. This paper adopts the “Diamond Princess” incident as a case study, examining how such public health emergencies pose challenges to international laws, particularly when they occur within the context of a cruise ship. The paper argues that cooperation on global health emergencies will continue to be a challenge until responsibility is more clearly allocated among stakeholders. Additionally, the paper formulates three principles for enhancing governmental cooperation, namely the fairness principle, the nationality principle, and the principle of common but differentiated responsibilities. It is advisable to carefully consider these key principles when reevaluating the international laws on public health emergencies in the post-COVID era.