The first thing you will want is an excellent quality fully waterproof metal detector. These range from a few hundred dollars to several thousand dollars. They also have various levels of waterproofing. Almost all metal detectors have waterproof search coils. But on many models, the control box is not waterproof. Waterproof metal detectors can be fully submerged from about 10 feet to a whopping 250 feet. First, you need to decide how deeply you will be searching. For deep diving, you will need a detector that is waterproof up to the maximum of 250 feet. You may consider a handheld detector as they are easier to maneuver underwater. The Pulse Dive by Nokta Makro is waterproof up to 200 feet and handheld. It is a great option for underwater detecting. You will also want something to contain anything you find and keep it safe while you dive. Ensure you are fully Scuba trained for deep dives.