When you want to go for under water gold and treasure hunting, then a you will need to choose an underwater gold detector. Only but first you need to decide whether your gold hunting expedition is going to be at the beach, creek, lake or the deep sea as an underwater gold detector will have to be chosen accordingly. Shallow water machines will not work for the deep sea expeditions. And a machine will not perform the same in salt water as lake water.
These are available in partially and fully submergible varieties depending upon your need. For Instance,
An underwater gold detector with only the coil being water proof cannot used for underwater gold hunting as only the coil can be submerged. If you practice metal detecting for gold in creeks then detectors with a fully submergible coil and control box is the correct choice. But if you going deep sea diving then you must choose a detector that has fully submergible coil and control box and is marked as suitable for deep sea gold hunting. Also you can classify them based on their technology where some underwater gold detectors are based on pulse induction technology and others are based on very low frequency .
These are based on detecting gold /metal on electronic pulses that sense precious metals quickly while not getting affected at all by salt or the minerals. So these are great for salt water. And they can also be used with deep sea expeditions.
These can operate at lower frequencies and can discriminate between junk and relics ,coins etc. However they are sensitive to ground mineralization signals. Ground balance control can but help to minimize this effect.