The original Explorer and Explorer II stock coils are constructed as follows.
There is an outer plastic coil housing (top and bottom) that's glued together, this housing is hollow and there is quite a bit of air space inside. Inside this outer plastic coil housing is another plastic shell that's molded into the shape of a double D coil. The coil's copper windings are placed in this shell and covered with epoxy. Then a thin wire is wrapped a few times around this coil shell, this is the shielding drain wire to ground. The entire inner coil shell including the drain wire is then painted with carbon black shielding paint. This is electrically conductive paint that's basically an anti-static shield, any kind of static is conducted through the shielding paint to the drain wire and off to ground. Without this shielding paint and drain wire the coil will false badly at the slightest touch, touch it with your hand, brush some grass on it and it will false. With the shielding paint and drain wire in place its perfectly stable. This inner coil is suspended on some foam pads inside the outer plastic coil housing.
Now, how the heck does water get inside this thing. Well if you look closely you will find bubbles in the glue and there can be pin holes where its not sealed. Then there is the coil cable entry, you have a coil cable flex fitting there to seal that hole, and if I remember correctly these don't have even a rubber o-ring but don't quote me on that. But still the coil is pretty well protected from water, rain, splashes, etc. so how is water getting inside the coil through these tiny openings? Okay what happens to air when its heated, it expands. Go detecting with your coil in the hot summer sun and the air inside the coil housing heats up and expands, venting out these areas. Then what happens when you go dunk your coil in cool ocean water? The air cools and shrinks creating a vacuum drawing salt water into the shell through the openings, salt water is electrically conductive and your coil will start wigging out. It doesn't like fresh water in there either. Capacitance is at work as well its not just static electricity.
One dunk in the ocean generally isn't enough to ruin a coil but repeated dunkings and your luck eventually runs out. Once the coil dries out it generally will operate normally again. You should have good luck if it was fresh water. But salt water is corrosive, it will eat at the drain wire and eventually the shielding paint/drain wire is compromised and your coil is toast.
