Unmanned surface vehicles (USVs) must detect small obstacles under glare, wave motion, poor visibility, and limited onboard computation. This review connects these sensing conditions to dataset selection, deep learning methods, multimodal fusion, and edge deployment. We distinguish direct USV evidence from transferable results and compare public datasets by acquisition viewpoint, target scale, sensor coverage, and annotation task. An accompanying benchmark evaluates YOLOv8s, YOLO11s, YOLO26s, and RT-DETR-L on FloW, SeaDronesSee v2, and WSODD using three training seeds, separate checkpoint-selection and assessment subsets for the new SeaDronesSee runs, and recall at matched false-positive rates. RT-DETR-L achieves the highest mean mAP50–95 under native preprocessing, with desktop-GPU latency 3.2–3.9 times that of the YOLO models. On FloW, stretching raises YOLOv8s and YOLO11s to 46.0% and 45.5% mAP50–95, respectively, compared with 45.5% for RT-DETR-L, showing the importance of preprocessing in model comparisons. Size-based recall and per-object diagnostics reveal different patterns of missed targets, localization error, and class confusion. The synthesis identifies priorities for preserving weak target cues, using temporal and complementary sensor observations, and evaluating detection quality together with device-specific latency and power.