Maritime security challenges are intensifying globally, from armed attacks on shipping in the Red Sea to critical infrastructure sabotage and environmental threats from shadow fleets. These developments have led some to question whether the United Nations Convention on the Law of the Sea (UNCLOS) remains ‘fit for purpose’. This article reframes that debate, suggesting that maritime security operates through a complex assemblage of institutions and mechanisms. In this context, UNCLOS is best understood not as a monolithic ‘constitution’, but as one important site in a more complex and dynamic system of maritime security governance. It examines how the United Nations (UN) system, regional organisations and informal arrangements collectively address maritime security challenges, revealing both the strengths and limitations of this diverse governance landscape. The analysis shows that while this complexity enables flexible responses to varied challenges, it also creates coordination problems and accountability gaps. Rather than calling for a revision of UNCLOS, the article proposes three pathways for recalibrating the system: establishing a dedicated UN maritime security body; addressing interface conflicts through targeted interventions; and selectively formalising successful informal mechanisms.