This article examines how maritime hybrid coercion in the Baltic Sea is framed, interpreted, and deterred across North Atlantic Treaty Organization (NATO), European Union (EU), and regional policy documents as well as selected Russian official narratives. It asks how recent public sources conceptualise critical undersea infrastructure incidents, shadow-fleet activity, and maritime ambiguity, and what their comparison reveals about deterrence, attribution, and escalation. The study employs qualitative comparative document analysis and structured close reading of three source clusters: analytical assessments of the Baltic maritime security, NATO, EU, and regional policy outputs, and Russian official doctrines and statements. The analysis is organised around four indicators: coercive mechanisms, deterrence mechanisms, escalation dynamics, and narrative frames. This design allows the article to distinguish evidence of events, evidence of policy adaptation, and evidence of public legitimation or counter-framing. The findings show that the Baltic maritime hybrid coercion operates through the interaction of material disruption, legal and attributional ambiguity, and narrative contestation. Critical undersea infrastructure incidents and shadow-fleet dynamics should be analysed together because both exploit commercial opacity, uncertain intervention powers, and contested public attribution. The article finds that effective deterrence depends not only on naval capability but also on resilience, surveillance, attribution, rapid repair, regulatory adaptation, and credible strategic communication. The Baltic case shows that maritime hybrid coercion operates through infrastructure disruption, legal and attributional ambiguity, and narrative contestation. NATO, EU, and regional actors are adapting through surveillance, resilience, regulation, and legal coordination, but deterrence by punishment remains limited by attribution problems, unclear intervention authority, and escalation risks.