The present paper presents the Reliability, Availability and Maintainability analysis performed by the authors in the framework of the H2020 REALTIDE project on two tidal turbine concepts - “complex bottom fixed tidal turbine” and “floating multirotor tidal turbine” (herewith called concept 1 and concept 3 respectively). The objective was to assess their designs and conditioning monitoring to enhance their reliability and availability performances. The two concepts were modelled by means of Monte-Carlo based software in order to study the impact of their failures to operational availability, and to determine the most critical components to unavailability. The study calculated an availability of 72.8% for concept 1 and 80.1% for concept 3; and the most critical components were similar for both concepts: Gearbox and High-Speed Shaft, Power Electronic Converter, Pitch System, Yaw system, Control System, Blade, and Generator. In both cases, several design modifications and monitoring were proposed and simulated in alternatives cases. The alternative case defined to maximise the turbine availability proposes a full set of implementations as the simplification of the tidal turbine design, redundancy of the Power Electronic Converter and Control System, and monitoring of Blades and Generator. The resulting availability of the tidal turbine is increased by +14.21% (concept 1) and +9.30 % (concept 3) comparing with the base cases. The influence of the weather and maintenance logistics were simulated in sensitivity cases and will be also presented in this paper.