A foundation structure (10) for locating an object (14) mounted on the foundation structure (10) on a sea bed or a river bed comprises an integral buoyancy device (22) having variable buoyancy and a rigid control surface (23). The integral buoyancy device (22) is movable relative to the foundation structure (10) to vary the position of the rigid control surface (23) and to control the position of the centre of buoyancy relative to the position of the centre of gravity. This stabilises the foundation structure 10 during submergence and surfacing. A method for controlling the deployment and retrieval of the foundation structure (10) is also described.