A water box for use in cooling a hot rolled product in a rolling mill comprises a plurality of nozzles arranged sequentially along a first axis. The nozzles are subdivided into base and top sections, with the top sections being adjustable between closed positions coacting with the base sections to define tubular enclosures through which the product is directed, and open positions. A clamp support extends in parallel relationship to the first axis, and carries clamp members. An operating mechanism adjusts the clamp support between an open setting at which the clamp members are separated from the nozzle top sections and the nozzle top sections are adjustable to their open positions, and a closed setting at which the top sections are in their closed positions and the clamp members are in contact with and urge the top sections into their closed positions.