Underwater acoustic channels are affecting by many factors like time varying multipath propagation, Doppler spread, and salinity, which can greatly limit the quality of data rate and transmission distance. Underwater optical communication represents a crucial technology that supporting higher data rate, secure links and low latency. In his paper the use of 64- Quadrature Amplitude Modulation (64-QAM) in underwater optical communication systems is investigated. The system is designed to offer higher data rate than those of acoustic communication system. The proposed design achieve data rate of 10 Gbps over a challenging transmission distance of 50 km. In our study, a simulation technique is used to evaluate the performance of the proposed design in the underwater conditions, focusing on factors such as Bit Error Rate (BER), constellation diagram and Quality factor (Q-factor). The achieved BER is of about 10-10 with a minimum signal to noise ratio (SNR) of about 30dB. This paper provides valuable results for the design of next-generation underwater communication systems for applications such as pollution monitoring, oil control and oceanography research.