Abstract: A device for indicating the presence in water of an object consisting of material capable of being affected by a magnet, the device containing a magnet. The device enables a reliable and effective indication to be given of metallic objects which are present in water. The magnet exhibits an active surface capable of interacting with the object in question and a floating body connected to the magnet in such a way as to be capable of being released from it, in conjunction with which there is present at least one ejector protrusion projecting beyond the active surface in question, so arranged that the ejector protrusion will endeavour, because of the nature of the interaction between the ejector protrusion and the object in question, to break the bond between the magnet and the floating body when the magnet is affected by the object in question.