In this paper, a high-speed and real-time underwater wireless optical communication (UWOC) system based on orthogonal frequency division multiplexing (OFDM) is designed and demonstrated using the field programmable gate array (FPGA) with a miniaturized demo board designed and made by ourselves. Through the parallel signal processing mode (i.e., our self-designed 8-path parallel radix-22 FFT/IFFT module) and the utilization of cyclic suffix (CS) instead of cyclic prefix (CP), the throughput and delay of the digital signal processing (DSP) are improved. Moreover, a low-complexity pilot-aided clock synchronization (PAS) scheme is proposed to solve the transmission errors induced by the frequency offset between the transmitter and receiver. The implementation details, as well as the analysis of resource utilization and latency, are presented. The feasibility and effectiveness of the designed real-time FPGA-based UWOC system in different turbidity waters is experimentally demonstrated. The results show that the proposed PAS scheme greatly reduces the bit error rate (BER) when the frequency offset is within ∼1.57 ppm. Furthermore, 16.3-m/ 2-Gbps and 14.1-m/ 4-Gbps real-time underwater transmission are successfully achieved, which to the best of our knowledge, is the highest data rate in real-time UWOC systems that has ever been reported, and the overall latency of the UWOC system is as low as 0.92 µs. The designed high-speed real-time UWOC system foresees a bright future in underwater applications over short to moderate distances.