For operations in complex underwater environments, bio-inspired robots offer manoeuvrability, stealth and autonomy. They integrate propulsion and control systems into one multi-purpose undulatory propeller. By generating large counteracting forces, undulating fins generate a wide range of net torques and accelerations. Bio-inspired designs can provide stealth by imitating the motion patterns of organisms. Bio-inspiration provides strategies to integrate multimodal sensory information, and algorithms on how to form robot swarms. Multimodal sensing and swarming facilitate navigation during indeterminate tasks, such as surveying or maintenance, provide robot redundancy, and increase the temporal and spatial resolution of a mission.