Orthogonal frequency division multiplexing (OFDM) techniques are attractive for underwater acoustic (UWA) communications due to their robustness against large multipath spread. However, OFDM systems in UWA channels suffer from intercarrier interference (ICI) caused by the Doppler effect. In this paper, we propose an effective mirror-mapping-based ICI self-cancellation method in OFDM UWA communications. Theoretical analyses and numerical results show that the proposed mirror-mapping-based ICI self-cancellation schemes can effectively reduce the carrier-to-interference power ratio (CIR) in comparison with the plain OFDM scheme in the additive white Gaussian noise (AWGN) channel with carrier frequency offset (CFO). To demonstrate their benefits for OFDM UWA communications, the mirror-mapping-based ICI self-cancellation schemes were tested in a recent sea experiment, conducted in Taiwan in May 2013. The decoding results show that the mirror-mapping-based ICI self-cancellation schemes improve the BER performance of the OFDM system considerably in UWA channels.