Underwater Acoustic (UWA) communications have to deal with channels that exhibit widely time-spreading multipath responses, together with Doppler spreading and limited bandwidth. These features seriously limit data throughput, even for sophisticated modulation schemes. Focusing on the large time-spreading characteristics, we analyze in this paper an OFDM UWA system such that the channel is longer than the Cyclic Prefix (CP) used. Under this condition, and considering an array of hydrophones at the receiver, we propose in this paper a Minimum Mean Square Error (MMSE) beamformer that makes use of two partial FFTs of half the original number of subcarriers to demodulate the OFDM signal. We show that one of the two demodulated signal can be received without Inter-Symbol Interference (ISI), even for longer-than-CP channels, and that the MMSE beamformer automatically is steered to direct and reflected paths obtaining an improvement of 4 dB in the Signal-to-Noise Ratio (SNR) and a reduction of 2 to 3 times the Bit Error Rate (BER) with respect to a conventional Delay-and-Sum (DS) beamformer.