Sorry, I thought you had written "UFO" dangers. Watch out for those anal probes. A real problem, unless you are a certain Florida congressman or an influential Evangelical Christian.
Unexploded devices that are completely flooded by saltwater are usually inert, though some can become lethal again when they dry out. Big stuff is almost always well sealed against water intrusion, and some smaller munitions as well. They are, after all, designed to be used at sea. A sure fire way of finding out if they still work is to strike them with a hammer. If you are killed, then it was still a live round. Since this is not a viable (intentional pun) option, it is far wiser to NOT TOUCH ANYTHING larger than a rifle cartridge, even if it's been submerged for years.
Primers are usually the most effectively sealed part of a shell. I inherited a WW1 'whizz bang" that had the propellant powder, contact fuse, and projectile explosive removed. It was a form of trench art, with all kinds of engraving on the brass shell. I examined it, and discovered that the big primer was intact! I disposed of it by giving it to my former wife.
