With the resource shortage in coastal areas, the construction of concrete structures using freshwater and river sand has brought great economic and environmental costs. The use of seawater, sea sand, and coral aggregates in concrete mixes has become an alternative solution for coastal and marine structures, especially for offshore structures and artificial islands. In this study, 378 seawater, sea sand, and coral aggregate concrete (Coral-SWSSC) specimens were prepared, and their mechanical properties were investigated comprehensively through compressive tests to explore the optimized mix design at different grades. Results showed that the mechanical properties of Coral-SWSSC were strongly correlated with the water-to-binder ratios, coastal particle gradings, and pretreatment method of coastal particles. Based on the experimental results, mix proportion designs of Coral-SWSSC were proposed for concrete from C20 to C50 grades, and the failure mechanism of Coral-SWSSC at different grades was discussed according to their respective failure modes. The findings of the current study provide knowledge on the optimized design of Coral-SWSSC, which can be used to promote the application of Coral-SWSSC in offshore, marine, and ocean engineering.