The durability of concrete in marine environments is critical for long-term structural performance, particularly in tropical climates where chloride-induced corrosion is a primary concern. This study evaluates the long-term performance of limestone calcined clay cement (LC3) concrete compared to ordinary Portland cement (OPC) concrete under natural exposure conditions in Cuba’s marine environment. Reinforced concrete elements were installed in a tidal zone and monitored over four years using durability indicators (DIs) such as carbonation depth, chloride profiles, air permeability, electrical resistivity, and chloride migration. Results demonstrated that LC3 concrete exhibited superior resistance to chloride penetration, lower air permeability, and higher electrical resistivity than OPC concrete, attributed to its refined pore structure and enhanced chloride-binding capacity. Despite LC3’s slightly higher carbonation rate, projected carbonation depths after 50 years remained negligible for both concretes. Additionally, LC3 showed better resistance to surface wear, further mitigating chloride ingress over time. These findings highlight LC3 as a promising sustainable alternative for marine applications, offering improved durability and reduced environmental impact.