To solve the problem of color cast and detail blur in low-illumination underwater images, a low-illumination underwater image enhancement algorithm based on white balance and relative total variation is proposed. According to the characteristics of selective attenuation of light in water, the algorithm firstly performs global illumination compensation for underwater images to improve image brightness and uses gray world algorithm to correct underwater image color. Then a new relative total variation constraint is constructed according to the edge preserving smoothness of guided filtering to estimate the illumination map. Finally, a reflection image, namely an enhanced underwater image, is obtained based on the low-illumination imaging model. The subjective effect, objective analysis, and application comparison experiments all prove the effectiveness of the proposed algorithm in correcting color cast and enhancing image details.