In this paper, frequency-domain equalization approach is proposed to deal with the intersymbol interference in underwater acoustic communications. To track the time-varying underwater acoustic channel, an adaptive algorithm is considered using overlap-and-save method. This technique makes it possible to remove the overhead due to the transmission of cyclic prefix over each block of data, such as in usual frequency-domain equalization systems. We compare both schemes through simulations to select the best one. The transmission scheme is validated in the Atlantic Ocean over a distance up to 2 kilometers in shallow water with high data rate single carrier QPSK communications (10 kbps). The receiver includes an efficient timing recovery tracking scheme followed by equalization. The results of simulations and experiments are evaluated in terms of bit error rate (BER) and mean square error (MSE).