First, trigger actors, by Unreal Engine. :)
How lights function in the Engine...
The exact steps to set a light to switch on or off based on if your "character" enters the room. You can modify this of course by changing what functions are used, or simply link the triggers to an actual light switch, just start the light as "off" and set the event trigger to set the light to "visible".
Adding water to the map is a bit more complicated than the light switch, so I have included the link or URL to the full tutorial for that below, it has video demonstrations and text that clarifies the steps for ease of use. Please let me know if you need anything else! :)