In order to conceive systems able to explore the underwater environment, the design of efficient underwater wireless communications is a major stake. However, the underwater acoustic channel is considered as one of the most difficult environments for communications, due to multiple limitations (frequency band, latency, Doppler effect…). Massive MIMO (multiple inputs, multiple outputs) systems, that consists of transmitting and receiving signals simultaneously from and to a large number of transducers is a promising approach to improve spectral efficiency enabling high data rate transmission. Due to the expensiveness of at-sea experimentations, it is crucial to model the underwater acoustic channel, making possible to develop new communication techniques. We here present a channel model for underwater acoustic massive MIMO (multiple inputs, multiple outputs) systems. This model takes into account the spatial coherence of the channel, which has a major impact on the channel capacity and on the methods that will be chosen for MIMO communications. An analysis on the impact of array correlation on the theoretical achievable rate is performed, using Shannon’s capacity.