NOVELTY - The platform, which is to be tethered to an underwater anchorage (8), comprises elongate surface-piercing spar buoys (1,2) linked by cross arms (4) for supporting the turbines (3). The platform is suited for rollover transition between a floating maintenance condition and submerged operating condition by asymmetric ballasting of the buoys. The buoys have independent buoyancy chambers along the length. Triangulation struts (6) provide for angular trimming control of the platform. The arms are mounted on one side of the buoys so as to be clear of the water when in horizontal floating condition.    USE - Platform for underwater turbines, including deep water turbines operable in fast-flowing tidal streams.    ADVANTAGE - Drag on water surface is reduced while the platform is provided with a catamaran-like form, by making the cross arms clear of water when in the horizontal floating condition. Roll stability is improved in the floating condition due to lateral spacing of the buoys. The platform can execute transition between maintenance and operating conditions in reliable, safe and predictable manner by preferentially biasing the attitude to the platform to assist transition between both conditions. Ballasting one side of the platform progressively in advance of the other side realizes a more controllable and predictable transition. Differential ballasting of the platform ensures preferred rollover characteristics.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method of transitioning the platform;    (2) a method of changing the attitude of the platform; and    (3) a method of changing the height of a turbine mounted on the platform.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic diagram of a tidal turbine platform in the upright operating position with turbines mounted on hydrofoil-shaped cross arms supported by twin surface-piercing spar buoys.    Spar buoys (1,2)    Turbines (3)    Cross arms (4)    Triangulation struts (6)    Underwater anchorage (8)