It is well established that the performance of communication systems is improved when deploying multiple antennas at one or both of the transmit/receive sides; and the amount of the gained improvement depends on the level of multipath richness of the propagation channel. In ship-to-ship overwater channels, quantifying such an improvement is not an easy task due to the effect of evaporation duct on imposing complex range-and height-dependent patterns of the received signal level. In this study, based on the parabolic equations (PE) method and using realistic evaporation duct distributions, we conduct extensive simulations in order to quantify the link-level improvement achieved when using 2 × 2 vertically-spaced Multiple-Input Multiple-Output (MIMO) systems against 1 × 2 Single-Input Multiple-Output (SIMO) and Single-Input Single-Output (SISO) systems. Then, we analyze the implication of such link improvement on the performance of a system comprised of a network of randomly distributed ships. When evaluating the outage throughput at the 2 nd percentile using realistic system parameters, it was found that 2 × 2 MIMO-MRC (maximum ratio combining) systems with 1 m antenna spacing are able to improve the outage throughput by threefold compared to SISO systems. This improvement increases to one order of magnitude when the antenna spacing increases to 10 m. It was also found that, in all cases, assuming using the same vertical spacing, 1 × 2 SIMO-MRC systems capture about 60% of the improvement achieved by 2 × 2 MIMO-MRC systems. On the other hand, 1 × 2 SIMO-DIV (diversity combining) systems are very sensitive to antenna spacing, and when assuming using the same vertical spacing, they can capture from 20% and up to 55% of the improvement achieved by 2 × 2 MIMO-MRC systems if the antenna spacing increases from 1 m to 10 m, respectively. INDEX TERMS Evaporation duct, multiple-antenna ship-to-ship communications, radio-wave overwater propagation.