As an ex police diver and dive supervisor I can only give you the wisdom of my experience diving in the south of England using metal detectors. The ones we trialed tended to indicate metal almost continually. The problem is that all bodies of water even remotely near human habitation will have had bottle tops, scrap iron, tin cans etc thrown into them for many years. Unless you have a detector that can differentiate between M42'S and bean cans you are going to become very frustrated. We also found huge quantitys of munitions in the water from German incendiary and HE bombs to home guard post war dumps of mortar rounds many in a highly unstable condition. I can only urge caution if you do search underwater in areas likely to contain unexploded munitions. the best method would possibly be to use a jackstay pattern moving very slowly and if viz allows making maximum use of the MK1 eyeball prior to touching anything.