Disclosed is a system that performs inspection and repair of an underwater structure, specifically core internals in a pressure vessel. An underwater vehicle is provided with a drive wheel, a contact force generating unit (e.g., thruster) that presses the drive wheel to a vertical wall of the underwater structure, an ascending force control unit (e.g., ballast) that controls the buoyancy of the vehicle, a guide roller that engages with and is pressed against an undersurface of the underwater structure by the ascending force controlled by the ascending force control unit. The underwater vehicle is appropriately positioned with respect to the vertical direction by the guide roller. The drive wheel is driven for rotation so that the underwater vehicle runs horizontally along the vertical wall to reach the inspection or repair site, where the inspection or repair is carried out by work tool mounted to the underwater vehicle.