Abstract: To address the scarcity of freshwater and river sand resources and the poor corrosion resistance of conventional concrete in marine island and reef engineering, this paper develops a basalt fiber-reinforced seawater coral sand geopolymer concrete (SWCSGC) using seawater and coral sand as raw materials. The evolution of mechanical properties and the underlying microscopic mechanisms under sulfate dry-wet cycles are systematically investigated, and a piecewise stress–strain model incorporating the fiber characteristic value is established. The results show that after 30 sulfate dry-wet cycles, the compressive strength of ordinary Portland cement concrete (OPC) continuously decreases, with a total reduction of 16.63%. In contrast, SWCSGC exhibits a sustained increasing trend in strength; notably, the group with 0.5% fiber content achieves a strength increase of 12.83%, demonstrating excellent resistance to sulfate attack. Microscopic analysis (XRD, SEM) reveals that the superior corrosion resistance of SWCSGC originates from the formation of stable reaction products such as calcite, hydrotalcite, and Friedel’s salt. Meanwhile, basalt fibers form an effective bridging network within the matrix, inhibiting microcrack propagation at the coral sand–matrix interface and retarding structural degradation. By introducing the fiber characteristic value λ = φ·(l/d), a quantitative quadratic polynomial relationship between λ and both peak stress and peak strain is established. A piecewise mechanical model suitable for SWCSGC before and after sulfate attack is developed, which fits the ascending branch well (R² > 0.99). Leveraging locally available seawater and coral sand, this material offers significant low-carbon and economic advantages over its full life cycle, providing a novel high-performance, long-durability composite solution for island and reef construction.