What beachs are you hunting?
I'm using my Sandshark and having a great time with it. It really loves iron though. If I don't get the beeping treasure by the 4th scoop I continue on. When it keeps getting louder the deeper I dig it usually turns out to be rusty iron from busted up crab traps over a foot deep. I'm usually hunting in 3-4 feet of water, I don't like dry sand.
Also, I look at the condition of the coins and bottle caps I'm picking up. It kind of tells me how old the stuff is I'm finding. When I find a really old crusty bottle cap one scoop down I know I'm working a 'fresh' area that hasn't been hunted out. The new clean bottle caps tell me I'm in fresh drop areas that may get hunted more frequently.
Good luck with the sandshark.