Single-input, multiple-output (SIMO) shallow underwater acoustic channel response measurements are used to develop baseband equivalent, discrete-time, intersymbol interference (ISI) SIMO channel impulse response models. Zero-padded orthogonal frequency division multiplexing (OFDM) is used with the database of ISI channel models to estimate the OFDM-constrained ergodic channel capacity, and the outage capacity. A simplified expression for the SIMO ergodic channel capacity is derived. The channel capacity is compared to estimates of bit error rate for differential binary phase shift keying (DBPSK) used in the OFDM SIMO system. Spread-response precoding is shown to significantly improve the OFDM/DBPSK performance.