Tidal energy is one of the most promising sectors of energy conversion that can be extracted from renewable sources, taking into account the uninterrupted flow of tidal currents regardless of the surrounding environmental conditions. In this case, a detailed analysis of the planned and unplanned maintenance attributes has been developed in order to examine the various O&M parameters influencing the cost elements. Major features for both planned and unplanned maintenance include the identification of the transportation, labor, workshop and equipment/tools cost. The above are estimated for different operational scenarios as well as for the maintenance of a single device per day. Overall, the O&M cost per device is estimated as well as the cost per MW (gross and net) and the cost/kWhr. The results show that the overall O&M cost is not prohibitive compared to other renewable energy applications while it may vary according to the initially selected O&M scenario.