The protection of critical maritime infrastructures has become a top political priority, since the September 2022 attacks on the Nord Stream pipelines in the Baltic Sea. This contribution reveals why the protection of infrastructures at sea is a difficult task. Reviewing the spectrum of maritime infrastructures (transport, energy, data, fishing, ecosystems) and the potential threats to them (accidents, terrorism, blue crime, grey zone tactics) demonstrates that designating infrastructures as critical and worthy of special protection measures is a political choice. The analysis moreover shows the need of protective instruments that are tailored to the specificities of maritime space, and the need for integrating diverse policy fields, including defense, diplomacy, marine safety, maritime security and cyber security. Cooperation with the infrastructure industry, enhanced surveillance and investments in repair capacities are also required.