At normal difficulty steel watchers are vulnerable to lightning. In patch 6 this got removed for higher difficulties. I'm assuming it's coded in a way, that you can't make them vulnerable again. You could probably consider this to be a bug.
Frost damage should still be increased against them when they are wet.
u 1 smart unga bunga!!11
I made the Steel Watch Wet and they became Vulnerble to Cold, hence indeed the coding seems to be ... innovative.
