In order to carry out high data rate communication using underwater wireless systems, highly efficient underwater antennas are required. The physical characteristics of conventional antennas are affected by lossy media, which are present in most underwater ecosystems. The MIMO antenna under consideration has been specifically engineered to address these identical factors. This multi-input multi-output antenna is designed on Rogers RT/duroid 5880 substrate with thickness h = 1.6 mm, the dielectric constant εr = 2.2, and tan δ = 0.0009. The overall dimension of the proposed antenna is 5 mm × 5 mm × 1.6 mm for a single antenna and 10 mm × 5 mm × 1.6 mm for a MIMO antenna having two elements. The patch of the proposed antenna is tilted by 30°, which provides a wide bandwidth for the UWB antenna. The proposed antenna exhibits an impedance bandwidth spanning from 4.7 GHz to 20 GHz, with port-to-port isolation exceeding 20 dB and peaking at 65.6 dB. Additionally, the simulation analysis explores essential MIMO diversity parameters, including ECC (Envelop Correlation Coefficient), DG (Diversity Gain), and TARC (Total Active Reflection Coefficient), revealing outstanding performance characteristics. This proposed MIMO antenna is an excellent candidate for underwater communication.