The performance of a floating oscillating water column wave energy converter depends on the variation of the pressure above atmospheric pressure in the pressure chamber above the water column. The pressure can be semi-actively controlled by the opening and closure of a valve between the pressure chamber and the generator. In this paper, a control is suggested, where the closure time intervals of the valve are taken as a fixed fraction of the peak period of a given sea state. The control relies on an estimation of the external wave loads, which, in turn, depend on the prediction of the future surface elevation in a given prediction interval, for which reason it is classified as a feedforward (open-loop) control strategy. A Kalman-Bucy filter has been devised for the indicated prediction. The optimal fraction of time of the sub-optimal controller with a closed valve is determined by comparison with the performance of the optimal control obtained by nonlinear programming. For a given sea state, it is demonstrated that the reduced performance of the sub-optimal controller is primarily related to the estimation error of the wave loads.