Apparently, large quantities of gold have been found in the Northern part of California, mainly in streams such as the Yoube Rivers and so on. These quantities of gold were the object of a lot of conflicts back in 1849, these being sought a long time before, when these devices didn't exist. In those times, there were others methods which were used, these being a lot more time-consuming and a lot more complicated, which makes these devices even more appreciated among people working in this domain.
Underwater metal detectors have allowed prospectors to discover a lot of such treasures not only in California, but also in other regions which hadn't been exploited before because of the lack of technological resources and opportunities. When it comes to those who can handle these devices, there is no need for professionals in scuba diving or for experts with such abilities. Also, the methods used today differ a lot from those which were used in ancient times, when such metal detectors were not known to mankind.
Therefore, even you can start using such an underwater metal detector if you think there is some treasure somewhere around you and you might also want to know that such a device doesn't require too much money, being totally affordable.