NOVELTY - The method involves lowering a working platform (30) for guiding a drilling tool to a water body bed (5). A drill-hole is introduced into the water body bed by the drilling tool. A tubular underwater-foundation element (10) is arranged and anchored in the drill-hole. A measuring unit is fixed on the foundation element. Position data of the foundation element is determined by the measuring unit. A position of the foundation element is set before anchoring by a positioning device e.g. hydraulic cylinder, on the platform via an adjustment head (40) based on the determined position data.    USE - Method for producing a tubular underwater foundation element i.e. underwater foundation pile, in a water body bed for anchoring a mast for an offshore wind power plant, and a flow turbine of a tidal power station in sea.    ADVANTAGE - The position data of the foundation element is determined by the measuring unit, thus accurately and directly determining the position of the foundation element. The position of the foundation element is set before anchoring by the positioning device on the working platform via the adjustment head based on the determined position data, thus positioning the foundation element at high accuracy. The method allows a mast, a system or a structure to be flange-mounted or fastened on the foundation element without the need for a compensating device, thus achieving stable and robust construction of the system. The method allows the measuring unit to be efficiently and releasably fixed on an upper area of the foundation element and to be removed after setting such that measuring unit is utilized multiple times for enabling use of measuring instruments of very high quality and high precision for the measuring unit.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) an adjustment head    (2) an underwater working arrangement.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an underwater working arrangement.    Water body bed (5)    Underwater-foundation element (10)    Underwater working arrangement (20)    Working platform (30)    Adjustment head (40)    Attaching device (50)