Under water acoustic communication using Multi Input Multi Output (MIMO) systems was shown to be effective in exploiting the time and spatial diversity characteristics of the underwater acoustic communication channel (UWAC). However, Inter Symbol Interferences (ISI) caused by the multipath phenomenon of the MIMO channel, affects the communication system performances. Our work shows that by using the Time Reversal (TR) technique, the channel capacity is achieved without the requirement for Channel State Information (CSI). A very shallow underwater sea trial was conducted. The results confirmed the validity of our analysis. It demonstrated that the 2×2 TR MIMO channel capacity is roughly 2 times larger than the non TR SISO channel capacity and is equivalent to the optimal MIMO system performances. Thus, our TR MIMO system (combined with a low level ISI mitigation) achieves the optimal MIMO system performances without the need for channel estimation.