You're theoretically right DM, energy is extracted from 'the wind' and in so doing, it is slowed down.
However, there's so much energy available, you'd need an awful lot of turbines before you slowed the wind by any significant amount.
Ditto waves. Note the recent complaint from surfers in Cornwall that their waves would be 'stolen' (reduced) by the proposed wave power plant.
(BTW, I'm not being pro wind turbine - in fact I don't think they'll ever produce enough power to be significant)