While it is not necessary to have a waterproof metal detector, if you will be metal detecting in areas with water, it gives you some peace of mind. Some detectorists like to shallow dive with their metal detectors in areas like rivers and streams. There can be a treasure trove of items in the bottom of rivers and streams, especially in areas where the water sources are used recreationally.  Even if you will not be diving, but want to detect in a very shallow stream, a waterproof detector helps you not have to worry so much about dropping the detector. If you fully submerge a non-waterproof metal detector, it can, and likely will, ruin the control box. Without the control box, your detector will not function. Regardless of the warranty, if the control box is water damaged, the company will not honor the warranty.