An adaptive bit loading algorithm is proposed based on visible-light pixelated multiple-input and multiple-output (MIMO) systems to deal with the problem of high number of bit errors for some subcarriers caused by the high-frequency attenuation, which is attributed to the absorption and scattering of channels and lens blur in underwater visible-light pixelated MIMO systems. By ensuring that the total number of bits allocated across all the subcarriers remains constant, the proposed algorithm dynamically selects the optimal subcarrier modulation order according to the signal-to-noise ratios of the subcarriers to reduce the number of bit errors for each subcarrier and improve the system reliability. The simulation results denote that the bit error rate can be considerably reduced when the transmission rate of the pixelated MIMO system is maintained constant.