The tut says "The water surface needs to have a Positive boolean, and the box needs a Negative boolean and the Null material we made before (so its surface won't interfere with your vision.) Group them to get the clear box for your camera; essentially a hole in the water. Put the camera into the hole." Im pretty sure it means that the water [which is positive] and the box[negative] once you group them it makes a hole in the terrain so basically the negative box cuts into the positive water which creates a "hole" and like it says put the camera where that negative box is. thats what i think.hope it helps??