This paper deals with a design methodology of permanent magnets (PM) generators used for fixed-pitch tidal turbines in a marine renewable energy context. In the case of underwater turbines, fixed-pitch tidal turbines could be very attractive and interesting to reduce maintenance operation by avoiding using such a complex electromechanical system for blade- pitching. In this technological case, one of the main control challenges is to ensure power limitation at high tidal current velocities. This control mode can be achieved using the generator flux-weakening. In this context, this paper proposes an original and systemic design methodology to optimize the generator design taking into account the tidal turbine power limitation for high tidal currents velocities.