handheld

If I bought a handheld underwater detector like the attached picture and I was diving just right offshore in 10 - 20 feet of water and I'm hoping to find gold/silver coins and gold/silver rings, what is the best operating frequency and why? Also, in the owner's manual it talks about frequency...

My Nokta PulseDive Underwater Metal Detector failed on me after going into the water (and not even using it). When I came back on land, although I could turn it on, I couldn't detect metal. Maybe some water got into it. I'm looking for something that is handheld so about 15" long. I would be...