SummaryThe available bandwidth of underwater environment tends to several kilohertz, which considers the main challenges of communications under sea water. On the other hand, the bit‐error‐rate (BER) performance degrades because of several reasons such as multipath propagation, time variabilities of the channel, attenuation, and water temperature. In this paper, we aim to improve the underwater acoustic (UWA) BER system performance by using orthogonal frequency division multiplexing (OFDM) based on fast Walsh‐Hadamard transform (FWHT) instead off fast Fourier transform (FFT). We proposed a low‐complexity equalization and carrier frequency offset (CFO) compensation for UWA‐OFDM–based FWHT using banded‐matrix approximation concept. Simulation results show that the UWA‐OFDM–based FWHT with low‐density parity check (LDPC) codes give a good improvement performance compared with traditional OFDM in UWA system especially in case of estimation errors.