I don't even remember the brand of the underwater metal detector I had about 30 years back but I do remember a trick that yielded a good amount of jewelry.
My dive buddy and I had both a metal detector and a suction dredge and someone, I forget who, told me that all the summer bathing beauties all slathered up in suntan oil, often lost their jewelry in the surf.
Given enough time, the surf picks up and drops both the jewelry and the sand with every wave, and eventually deposits them in a line along the beach. And it's pretty consistent as the dense jewelry drops first and makes the line.
So when the summer crowds, and the police, left after labor day, we could go out play in the surf. Using the metal detector one of us moved back and forth from the beach into the ocean in the surf zone. Once we had a hit on the detector, we brought the dredge which followed along. When we determined where the line of hits was it became easy.
The dense gold lined up parallel to the beach and you could snorkel along with the metal detector and when a hit sounded, dig at the spot with the dredge until the hit want away. We continued along the beach this way collecting everyones misfortunes.
We usually got over an ounce each outing and on one trip to a north shore beach got close to 3 ounces. Must have been the rich folks watering hole. At this one site there was a piece with a nice diamond which my buddy wanted because he and his lady were serious. So he got the stone, and the girl. He still has both so I guess it was a good thing.