Jeep-gold, I think you're making a mistake, to take up scuba, for the thought/notion of md'ing for casual beach-goer fumble fingers losses. ~20 yrs ago, I too had the same visions of grandeur you are having now. Thinking there's a "magical virgin beach" just out of reach of the poor saps who have to stay in the surf zone and higher, eh ? OH BUT I WISH IT WERE THAT SIMPLE.
You have to realize that in the same exact way that the wet-sand zone of the beach can be totally sterile ( because sand is coming "in", and no storms have eroded lately), SO TOO is the underwater sand world the SAME thing. In other words: the sand at the bottom of the beaches, out beyond the lowest tide marks, is every bit as susceptible as the sand ABOVE the surf line (wet zone of the beach). Thus simply because perhaps no one has ever detected there, does not bode for necessarily good detecting. There are "dunes" and moving sand underwater (probably MORE so) just like there is on the beach above. And if it's sterile, it's sterile !
Yes I went out and found a few coins and a silver earing in 10 ft. of water, but ........ got a quick dose of reality: The underwater time is very expensive, and time-consuming. Ie.: to get a single hour underwater, is much preparation, cost, suiting up, cumbersome, slow, etc... And once you're down there, you're fighting currents that keep you from staying on a spot while you try to dig. And the minute you start digging a target: Clouds of silt you've just disturbed start billowing up, so now you can't see. But you struggle persist, trying to stay on target, going deeper and deeper toward the beep. Finally after 5 frustrating minutes: WALLAAH! a soda can. After an hour, you have 3 coins and an earing.
All of the sudden it dawns on you: I could have done better simply detecting the beach above. Especially just waiting for mother nature to erode, and then you can walk freely around finding the producing zones, without expense, hassle, etc..
The only exceptions I can think of for scuba @ fumble-fingers regular beach swimmer losses, would be: fresh-water swimming holes. Or cove beaches not-susceptible to underwater currents. And preferably, of course, riddled with swimmers in a warm water environment (like under neath dive platforms, etc....). But for the open facing ocean like we have here in CA ? No, sorry.
It's a lot of fun (seeing the fish, etc...), but for md'ing, it's not what you're envisioning it to be.