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Adaptive bit and power loading is a constraint optimization problem with generally two cases of practical interest, where the objectives are the achievable data rate maximization (RM) and system margin maximization (MM). In this paper, we propose a different optimization model for underwater acoustic (UWA) channels, which is achieved by two algorithms: one is the bandwidth-efficient bit loading algorithm; the other is the Lloyd algorithm based limited feedback procedure. It aims at minimizing the power consumption with constraints of the constant symbol data rate and desired bit-error-rate (BER). The optimization result is derived by the Lagrange multiplier method. The Lloyd algorithm is employed to quantize the CSI at the receiver and construct the codebook, which is adopted to achieve the limited feedback process. After selecting an initial bit loading vector upon the current CSI, the receiver will broadcast its index to the transmitter, then the transmitter will compute the bandwidth-efficient bit loading algorithm and allocate the corresponding power and bits to each subcarrier. Simulation results will reveal the benefits of adopting these two algorithms from different aspects.