Sorry for the delay, been dying to try it for the last week and a half and needed to give it a good test! Works AWESOME, I used this to take a layermask Nothing and make it everything besides Water:
Not sure if this is the best way of doing it, but it works for me! This means I never have to worry about adding new layers and going through all of my old layermasks, I can just remove the layers I need excluding at the time. Thanks so much!