Oceans are an environment attracting growing interest, with major economic stakes. There exists a need to design systems able to explore the underwater environment, one of the challenges being the ability to communicate undersea. The underwater acoustic channel is considered as being one of the most difficult environments for designing communication systems, due to multiple limitations (frequency band, latency, Doppler effect...). A method that could allow an increase of the data rate is the use of massive MIMO (multiple inputs, multiple outputs) systems, where many transducers are used to emit as well as to receive the signal. The objective of this paper is to present a model for a massive MIMO underwater acoustic communication channel in order to carry out a preliminary analysis of the impact of array correlation on the theoretical achievable rate by using Shannon capacity.