All metal detectors work fine on dry sand. And most models have waterproof coils that can be used underwater. Note, however, that displays and control panels are usually not waterproof, so if you want to dip your tool in the water completely, you should look for specific models that are totally submersible. Also, note that not all detectors will be able to find anything under salty seawater as it has a different mineral balance, so ground balance settings of your detector will have to be changed.