Abstract: In orthogonal frequency division multiplexing (OFDM) systems, the differential modulation inherent in block differential encoding enables low-complexity and robust communication without requiring channel state information, while the coding process provides diversity gain that can effectively enhance the performance against fading. However, this scheme still faces challenges such as strong time-frequency double selectivity and low signal-to-noise ratio (SNR) in underwater acoustic channels. This paper proposes an OFDM communication system based on block differential encoding for underwater acoustic communications. At the transmitter, a frequency-domain block differential encoding scheme is employed. To enhance system reliability under low SNR conditions, it is concatenated with symbol-based Turbo codes. A method is proposed to calculate non-binary differential symbol log-likelihood vectors based on estimated subcarrier correlation coefficients, addressing the degradation of correlation between adjacent subcarriers in time-frequency doubly-selective channels. The simulation results demonstrate that the proposed scheme has 4 dB and 2 dB gains compared to differential phase shift keying (DPSK) scheme and phase shift keying (PSK) scheme at the same rate when the spectral efficiencies are 0.5 (bit/s)/Hz and 1 (bit/s)/Hz, respectively. The sea trial results demonstrate that the SNR thresholds for correct decoding of the proposed scheme are 7 dB and 11 dB when the spectral efficiencies are 0.5 (bit/s)/Hz and 1 (bit/s)/Hz, respectively.