This paper presents the design and implementation of a wave excitation force estimator based on the cubature Kalman filter (CKF) for a CETO 6, a submersed wave energy converter controlled using three power take-off systems attached to the tether lines. Wave excitation force estimation is required by many advanced wav energy converter (WEC) control approaches. However, contrary to the single degree-of-freedom (1-DoF) case, the literature on the design of wave estimators is quite scarce for devices with multiple degrees-of-freedom. The advantages of CKF wave estimator described in the paper are its performance and ease of tuning when working with a high-order system, as it is the case for the 6-DoF linear model used for the design. Preliminary results for all the 6 directions are presented in nominal conditions and also in mismatch conditions, where the estimator is applied to a full nonlinear model of the system, under the action of long-crested and short-crested waves.