Abstract: Underwater acoustic networks (UANs) feature narrow bandwidth, large propagation delay and limited energy, which bring formidable challenges to reliable underwater data transmission. As an emerging rateless coding scheme, online fountain code (OFC) is capable of formulating optimal encoding strategies according to decoding status feedback. Compared with conventional fountain codes, OFC possesses online adaptability and lower encoding overhead, and has therefore been increasingly applied to reliable data transmission in UANs. To systematically elaborate the research advances, core technical bottlenecks and future development trends of OFC and its applications in UANs, a systematic review and quantitative analysis of 23 typical OFC mechanisms are conducted in this paper. The core principles of OFC adapted to UAN scenarios are analyzed and the above 3 research dimensions are comprehensively summarized. The results reveal that OFC, owing to its distinctive superiorities, possesses great application potential in tackling reliable data transmission issues in UANs, and can provide solid theoretical guidance for further in-depth investigations on OFC-based UAN communication.