Integrated bridges of today’s vessels are complex and distributed maritime systems that interconnect versatile electronic equipment. However, digitized vessels have long ceased to be isolated systems and are thus increasingly vulnerable to cyber attacks. In this context, integrity and authentication of the communication onboard is crucial. Therefore, we introduce MARMAC, a low-cost solution to retrofit authentication of nautical communication. MARMAC is based on symmetric cryptography and extends the prevalent IEC61162-450 protocol enabling a backward-compatible solution which mitigates common attacks. Using a specific gatekeeper approach, MARMAC can prevent unauthenticated messages from being processed that could otherwise affect the nautical situational awareness on the bridge. Our approach is evaluated using real network traffic in a laboratory testbed with low-cost hardware, highlighting its feasibility and potential to secure existing maritime systems.