This paper deals with the study of an unconventional design of marine tidal turbine where the electrical generator is located in the periphery of the blades and where the magnetic gap is underwater. This kind of solution called "RIM DRIVEN" structure allows increasing the compactness and the robustness of the system. Due to the strong interaction of the multi physical phenomena, an electromagnetic model and a thermal model of the PM generator are associated with a hydrodynamic model of the blades and of the water flow in the underwater air gap. These models are used in a global coupled design approach in order to optimize, under constraints, the global efficiency of the system. This approach is illustrated in a case study which deals with the design of a 10m diameter tidal turbine. Proposed coupled models are validated by comparison with experimental data from the tests of an academic low power demonstrator.