Using M-ary quadrature amplitude modulation (M-QAM), the bit error rate (BER) performance of an underwater optical wireless communication (UOWC) system is examined in this research for a variety of water conditions, including a pure sea, clear ocean, coastal water, and a turbid harbor. Attenuation loss, geometrical loss in the case of a semi-collimated laser source, and turbulence, which affect underwater communication, have been taken into consideration for the first time. Taking into account geometrical and attenuation losses, the average BER for the newly proposed underwater channel was derived and expressed in closed form using the lognormal turbulence model. The effect of this underwater channel on coverage and reliability has been illustrated using simulations in MATLAB. With receiver diversity, efforts to enhance both range and reliability have been formulated and evaluated. Moreover, our results show that receiver diversity can improve performance defined in terms of reliability and range especially during weak turbulence, a 1×2 single-input multiple-output transmission on a 30 m pure sea water link with a log-amplitude variance of 0.9, for instance, can result in a 5 m improvement at a BER of 10−4. The proposed system will aid the defence industry especially inter-submarines communication.