1. A submersible robot for operating a tool relative to a surface of an underwater structure, comprising: a support assembly; a tool holder for holding the tool in operative position relative to the surface of the underwater structure, the tool holder being movably mounted on the support assembly so that the tool is movable in a work area relative to the surface of the underwater structure when the tool is mounted on the tool holder; a driving arrangement mounted on the support assembly for moving the tool holder so that the tool is movable within the work area when the tool is mounted on the tool holder; a locking arrangement mounted on the support assembly for locking a position of the support assembly relative to the surface of the underwater structure; a levelling arrangement mounted on the support assembly for adjusting an orientation of the support assembly relative to the surface of the underwater structure when the support assembly is locked by the locking arrangement; a sensor unit directable toward the surface of the underwater structure, for measuring a distance between the robot and the surface of the underwater structure; and a programmable control unit mounted on the support assembly for operating the driving arrangement, the locking arrangement, the levelling arrangement and the tool and receiving measurements from the sensor unit, the programmable control unit having an operation mode wherein a positioning of the robot relative to the surface of the underwater structure is determined and controlled as function of an initial position of the robot for defining a first work area, and shifted positions of the robot for defining additional work areas, the work areas having overlapping portions with one another for tracking displacements of the robot relative to the surface of the underwater structure using the sensor unit.