| Summary: | The performance of <italic>M</italic>-ary orbital angular momentum-shift keying (OAMSK) modulation-based underwater wireless optical communication (UWOC) system is investigated over oceanic turbulence with Laguerre–Gauss beam considered. On the basis of Rytov approximation, the detection probability and power distribution among the received signals are derived. The conditional probability and symbol error rate (SER) are, then, achieved by the maximum likelihood estimation. With Blahut–Arimoto algorithm, the channel capacity for this UWOC system is obtained. The results show that the optimal transmitted OAM mode set <italic>S</italic> is mainly restricted by the interfering energy and decaying effective energy. With the optimal mode interval achieved, the SER performances for different modulation orders <italic>M</italic> and the minimum required transmitting power to reach the SER of <inline-formula> <tex-math notation="LaTeX">$P_{e} = $</tex-math></inline-formula> 10<sup>−9</sup> are sensitive to the variation in oceanic turbulence conditions. Additionally, the value of capacity will shift to a higher value with the increasing <italic>M</italic>, and a suitable <italic>M</italic> should be selected to balance the transmitting power consumption and capacity gain. This work is beneficial to <italic>M</italic>-ary OAMSK modulation-based UWOC system design.
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