This paper proposes a staircase joint optimization scheme (SJOS) with alternating diagonal interference cancellation and power allocation in an underwater wireless optical communication system based on nonorthogonal multiple access (UWOC-NOMA) with the multi-user paired. The scheme employs the directional iteration to alternatively optimize the subproblems of the interference cancellation and the power allocation. Furthermore, a one-way sorting algorithm based on the alternating diagonal interference cancellation and power allocation subalgorithm based on the conjugate gradient method are presented to solve the two subproblems, respectively. Simulation results show that the algorithm effectively reduces the average outage probability of the system with fast convergence, even with an increase in the number of paired users.