Submersible infrastructure, such as telecommunications cables and energy pipelines, plays a strategic role in modern maritime activities and regional energy security, especially in semi-enclosed seas such as the Baltic Sea. The increasing level of vulnerability of underwater installations raises legal concerns regarding the adequacy of regulation in the current international and national legal regimes. This research aims to examine the international legal framework governing the protection of subsea infrastructure and examine its application in the Estonian and Finnish national legal systems. This research utilises a normative legal research method with a statutory and conceptual approach, focusing on relevant international legal instruments, in particular the 1982 United Nations Convention on the Law of the Sea (UNCLOS), as well as maritime regulations and national criminal law. A comparative approach was used to explore the similarities and differences in the regulatory patterns applied by the two countries. The findings show that while international law provides a general normative framework for the protection of submarine cables and pipelines, its effective enforcement is highly dependent on national-level arrangements. Estonia and Finland both adopt international obligations into domestic law, but have different regulatory foci, particularly in the aspects of deterrence and criminal liability. These variations in approach have the potential to affect the level of infrastructure protection in transboundary maritime areas. This research concludes that it is not enough to rely on international norms for the protection of underwater infrastructure but requires harmonised national implementation and strengthened bilateral cooperation to ensure legal certainty and regional maritime security.