Advances in sensing and communication technologies have enabled emerging applications in ocean research. The stringent resource limitations, however, hinder local task processing on maritime devices. Cloud-based task offloading is also not feasible due to the unaffordable costs induced by satellite links. In this work, we propose a visionary architecture on software-defined maritime fog computing (SD-MFC), which adopts fog computing to provide localized services and uses software-defined networking technology to provide the abstraction of underlying resources. In SD-MFC, coastal infrastructures and large-size vessels are turned into maritime fog nodes to process the tasks generated by surrounding maritime devices. By exploration of a real-world vessel trajectory dataset, the feasibility and effectiveness of the proposed SD-MFC are verified.