Become fully geared, backpack and all (anykind) Have any weapon placed on your back, preferable a large one like an AK or a Mosin, M4 if you are lucky. Go to the large island north of Electro. (The one you usually pass when trying to get to electro I think its near Kamy?) Swim your way from the shore from the mainland to the island, and watch your backpack and gun (possibly more) plop down to the bottom of the sea. You can do this a few times to see how consistent it is but its so far happened every time ive gone across the island.