Yaw bearing and connection arrangement for an underwater turbine

A tidal turbine is mounted to a support structure 214 via a connection which can be clamped 250 to prevent movement, or partially released to allow rotation. A lifting device 252 such as a spring 254 separates the coupling surfaces to allow rotation, and a bearing 256 may also be provided. An active hydraulic lifting system may be used which supplies fluid to cavities 515 between the supporting surfaces. Alternatively one or more actuators may be used. Active control may be used to differentially lift the turbine, to tilt a yawing axis away from vertical, so that the turbine yaws under its own weight / gravity. This may be assisted by moving ballast, or by flooding or emptying ballast tanks.