This paper presents the analyzed results of an integration of an offshore wind farm (OWF) and a seashore wave farm (WAF) connected to an onshore large power grid using a flywheel energy storage system (FESS) to simultaneously achieve power-fluctuation mitigation and dynamic-stability enhancement. The performance of the studied OWF is simulated by an equivalent aggregated 80-MW doubly-fed induction generator (DFIG) while the characteristics of the studied WAF are simulated by an equivalent aggregated 40-MW squirrel-cage induction generator (SCIG). A frequency-domain approach based on a linearized system model using eigenvalue analysis is performed. A time-domain scheme based on a nonlinear system model subject to disturbance conditions is also carried out. It can be concluded from the simulation results that the proposed FESS can effectively stabilize the studied OWF and WAF under various disturbance conditions. The inherent fluctuations of the power injected to the power grid can also be effectively smoothed by the proposed control scheme.