Is there some reason why you can not use a Blue Switch (or Switchez) ... which are the accepted controllers ... for the water tiles? Or is it that you want the purple color for some graphic you are designing?
Why don't you test it out yourself to see if it works? That's an idea.
In the rules, it says green switch can rule bridge tiles and its not acceptable.
The intent of the game designers was that certain color switches always controlled certain things: green switch=green pyramids; blue switch=bridge tiles (water/other); red switch=red pyramids; purple switch=purple pyramids; yellow switch=two-way arrows, etc. Designers found that switches could, in fact, control things other than they were designed to ... I even created a Quest "Metamorphosis" which used Black One-Time Switches as if they were Question Mark Switches, with the result that things that changed ... stayed changed! But I also plainly stated that was not a desirable way to do things.