To enhance the visual quality, guided image filter is usually adopted to refine the crude transmission map of underwater images. In this paper, we first develop a simplified guided image filter to reduce the computational complexity of transmission estimation while maintaining good visual quality. Subsequently, a low-cost architecture for the simplified guided image filter is proposed to achieve real-time Full-HD (1920 × 1080) underwater image enhancement. The proposed architecture is synthesized with TSMC 90nm CMOS technology and the result shows that it can operate at 100MHz and support Full-HD image enhancement at a throughput of 30frame/s with 10.5K gate counts and 1.35KB on-chip memory. Compared to previous design, the proposed design uses 11.3% of gate counts and 42.2% of on-chip memory to achieve the same frame rate.