Underwater visible light communication (UVLC) systems suffer from a strong nonlinear effect and high inter-symbol interference (ISI). In this study, to improve the performance of a UVLC system under such conditions, we propose a novel nonlinear hybrid modulation scheme named two-dimensional bit allocation (2DBA). By comparing the performance of 2DBA with the famous Levin-Campello (LC) algorithm and the quadrature amplitude modulation (QAM)-based time-domain hybrid modulation (TDHQ) algorithm, we have proved by analysis and experiment that 2DBA can outperform the power allocation-based LC algorithm and the TDHQ algorithm below the 3.8×10-3 hard decision forward error correction threshold (HD-FEC) when the system has a severe nonlinear effect and ISI. The data rate 3.24 Gb/s of 2DBA is measured after 1.2 m underwater transmission; as far as we know, this is the highest data rate reported in a blue LED chip based UVLC system.