The paper deals with problems of selection of structures and optimum design of multipole permanent magnet synchronous machines (PMSMs) for operation in offshore wind- and wave-power plants under severe conditions for maintenance service. It is shown that multipole PMSMs are a new and very suitable electrical machine for direct connected turbogenerators in power plants and, at the same time, multipole PMSMs meet a number of other important demands, including high reliability, contactless construction, high efficiency, minimum mass and dimensions, and cost effectiveness. With the aim of systematization, an integrated classification scheme of PMSMs is presented to describe the variety of designs of stator and rotor configurations. With the help of a theoretical analysis based on a geometrical approach, analytical expressions for PMSM comparison are derived using the relationship between the criterion of optimization (maximum power density) and the main geometric parameters of PMSMs. Comparative analysis of the PMSMs with radial and axial magnetic fluxes for the case of various numbers of poles and different diameters of rotors has confirmed that axial multipole PMSMs are synchronous machines with maximum power density and an optimum variant for direct connected turbogenerator structures for offshore wind- and wave-power plants. It is noted that the advantage of axial PMSMs over radial PMSMs is monotonic growth in accordance with increasing number of poles p. Under the condition that p ≥ 20, which is strictly necessary for designing direct connected turbogenerators in offshore wind- and wave-power plants, we have more than doubled the power density at axial PMSMs as compared with radial ones.