This study investigates the feasibility of using Boat Fiber Waste Powder (BFWP) as a partial replacement for fine aggregates in M30-grade concrete. BFWP, derived from discarded glass-fiber-reinforced polymer (GFRP) boat components, was incorporated at 0%, 10%, 20%, 30%, and 40% replacement levels by mass of fine aggregate. Workability (slump test, ASTM C143), 7-day and 28-day compressive strength (ASTM C39), and 28-day water absorption were evaluated. Key findings are: (i) workability remained consistently high (slump 130–142 mm) across all replacement levels; (ii) water absorption decreased progressively from 1.02% at 0% BFWP to 0.75% at 40% BFWP, a 26% improvement; and (iii) compressive strength decreased from 25.81 MPa (control) to 20.54 MPa at 10% and 5.23 MPa at 40% replacement at 28 days. An optimal replacement level of 10–20% is identified, retaining 65–80% of control compressive strength while improving durability. BFWP is recommended for non-structural applications such as paving blocks and lightweight panels, offering a viable pathway to divert GFRP waste from landfill while reducing reliance on natural sand.