A watch including a watch case provided with a case middle, the case middle carrying an element provided with at least one index and defining an inner cavity. The watch further includes at least one pair of electrodes and at least one light source device connected to the electrode pair and arranged opposite the index, the electrodes of the electrode pair being fixed inside the inner cavity, the element being water permeable such that, when the watch is immersed in seawater, the seawater can penetrate the inner cavity so that the electrode pair and the seawater form an electrochemical cell, for powering the light source device.