A method for mounting at last one tidal turbine and its associated support structures in a river or sea bed. The turbine and associated support structures are joined to a base of such weight as to involve gravitational forces of such magnitude as to be sufficient to withstand forces acting to tend to displace the turbine and its associated support structures. The base structure can be pre-assembled and transported to a required location of installation by lowering to the sea or river bed. Conveniently, the transportation is by means of an installation vessel incorporating a facility for lowering the entire assembly including at least one turbine, a support column, and the base structure in a controlled manner to rest at a desired position on the river or sea bed.