Based on Mie scattering and Monte Carlo ray tracing method, the spectral transmission behavior of visible light in the seawater is estimated. Its optical model is constructed for the fast and accurate prediction of light quality changes of phosphor-based white LED underwater illumination with distance. Because of this, we propose to add a dual-wavelength LED to the original white LED, which would perform spectrum compensation at each illumination distance to restore its good light quality. The result shows that such a dynamic light source can successfully maintain correlated color temperature at 6000 K and color rendering index above 80 under different underwater illumination distances.