With an increase in the shooting depth, underwater images suffer from issues such as low brightness, color distortion, and blurred details. Therefore, an underwater low-illumination image enhancement algorithm based on an encoding and decoding structure (ULCF-Net) is designed. First, a brightness enhancement module is designed based on a half-channel Fourier transform, which enhances the response in dark regions by combining the frequency domain and spatial information. Second, cross-scale connections are introduced within the encoding and decoding structure to improve the detailed expression of underwater optical images. Finally, a dual-stream multiscale color enhancement module is designed to improve the color fusion effects across different feature levels. Experimental results on publicly available underwater low-illumination image datasets demonstrate that the proposed ULCF-Net exhibits excellent enhancement in terms of brightness, color, and details.