Abstract: A sitting bathtub has one lateral wall almost completely open. A hatch is placed over the opening so that one of the hatch's lower corners by the weight of the hatch rests against the side wall of the tub. A movable handle is arranged to press the rest of the lower part of the hatch against the side wall to obtain an initial seal. When water is filled into the tub, the hydrostatic pressure against the hatch climbs upwards forcing the sealing function successively to climb in advance upwards over the rising water level. The handle can be operated from inside as well as from outside of the tub. The force with which the handle presses against the inside of the hatch is proportional to the turning angle of the handle.