The use of untethered miniature swimmers is a promising trend, especially in biomedical applications. These swimmers are often operated remotely using a magnetic field commonly generated using fixed coils that can suffer from a lack of compactness and heating issues. The analysis of the swimming capabilities is still an ongoing topic of research. In this letter, we focus on the ability of a magnetic actuation system to operate the propulsion of miniature swimmers with flexible flagellum. As a first contribution, we present a new manipulability criterion to assess the ability of a magnetic actuation system to operate a swimming robot, i.e. to ensure a displacement in any desired direction with a fixed minimum speed. This criterion is developed thanks to an analogy with cable-driven parallel robots. As a second contribution, this manipulability criterion is exploited to identify the dexterous swimming workspace which can be used to design of new coil configurations as well as to highlight the possibilities of moving coil systems. A case study for a planar workspace surrounded by three coils is in particular carried out. The accompanying video illustrates the application of the proposed criterion in 3D, for a large number of coils.