I saw a zombie go to the bottom of a river in full gold and then his armor floated to the top. I guess drowned can't have armor so it just drops on the ground. now i have full durability gold armor. I don't have a screen shot of this happening because i haven't figured how to take a screenshot on windows 10. I also haven't tried seeing if it is a 100% drop rate either or if I was just super lucky to get the full set.