We show that unlike radio frequency channels, amplify-and-forward relaying can increase the data rate in non-fading underwater acoustic channels. For frequency-selective Rayleigh fading underwater acoustic channels, we focus on QPSK modulation and show that the data rate can be increased by using either decode-and-forward or amplify-and-forward relaying, with the former having a significantly higher performance. This fact indicates that rather than increase the transmit power to increase the data rate, a more power-efficient method involves increasing the number of relays and decreasing the path length between them. We also investigate power allocation methods and analyze their effect on the achievable rate. We show that distributing the power uniformly across a frequency band can match the performance of the rate-maximizing power allocation for QPSK modulation.