A submersible turbine assembly comprising: a frame sized and configured for supporting one or more flowing water driven turbines at a predetermined depth range below the surface of a water body having a bed, the frame including at least one fixed buoyancy component and at least one variable buoyancy component and optionally one or more hydrodynamic lift-generating surfaces, the one or more flowing water driven turbines being expected to produce a drag force which varies with water velocity, the assembly being arranged to be maintained in a predetermined position above the water body bed and submerged below the expected water body surface by at least two upstream taut mooring line runs (144b, 144c) and at least one downstream taut mooring line run (144a, 144d), wherein the mooring line runs are arranged to be anchored to the water body bed at respective anchor points (148a, 148b, 148c, 148d) spaced apart from a point below the expected position of the assembly by at least the height of the assembly above the water bed, the frame having attachment points for said taut mooring line runs comprising upper and lower attachment points for spaced apart attachment of upper (146a, 146b, 146c, 146d) and lower (145a, 145b, 145c, 145d) or primary and secondary mooring lines or cables forming each mooring line run, each mooring cable coupling the upper and lower attachment points to a single anchor point on the water bed, the attachment points arranged to permit movement of at least some of the mooring lines (144, 145, 146) relative to the frame during installation and having an arrangement for locking the mooring lines relative to the frame during use, the assembly being arranged so that in use in a water current having a nominal flow velocity of 3 metres per second both the upstream and downstream mooring line runs (144, 145, 146) remain taut and wherein the net upward force resulting from the fixed and variable buoyancy and any hydrodynamic lift minus the weight of the assembly is at least 25% of the drag on the structure and thrust produced by the at least one turbine. a frame sized and configured for supporting one or more flowing water driven turbines at a predetermined depth range below the surface of a water body having a bed, the frame including at least one fixed buoyancy component and at least one variable buoyancy component and optionally one or more hydrodynamic lift-generating surfaces, the one or more flowing water driven turbines being expected to produce a drag force which varies with water velocity, the assembly being arranged to be maintained in a predetermined position above the water body bed and submerged below the expected water body surface by at least two upstream taut mooring line runs (144b, 144c) and at least one downstream taut mooring line run (144a, 144d), wherein the mooring line runs are arranged to be anchored to the water body bed at respective anchor points (148a, 148b, 148c, 148d) spaced apart from a point below the expected position of the assembly by at least the height of the assembly above the water bed, the frame having attachment points for said taut mooring line runs comprising upper and lower attachment points for spaced apart attachment of upper (146a, 146b, 146c, 146d) and lower (145a, 145b, 145c, 145d) or primary and secondary mooring lines or cables forming each mooring line run, each mooring cable coupling the upper and lower attachment points to a single anchor point on the water bed, the attachment points arranged to permit movement of at least some of the mooring lines (144, 145, 146) relative to the frame during installation and having an arrangement for locking the mooring lines relative to the frame during use, the assembly being arranged so that in use in a water current having a nominal flow velocity of 3 metres per second both the upstream and downstream mooring line runs (144, 145, 146) remain taut and wherein the net upward force resulting from the fixed and variable buoyancy and any hydrodynamic lift minus the weight of the assembly is at least 25% of the drag on the structure and thrust produced by the at least one turbine.