If you ever get a problem in hammer where you see a phantom brush that is pure white somewhere in the back of your screen and don't know why, or how to get rid of it, then this should help!
I've found that this phantom white brush occurs when you open the Lightmap Grid while looking at a water brush in Hammer.
When you switch the Lightmap Grid back to 3d Shaded Textures, the phantom white brush will appear in the shape of the water that was being viewed. **note: this also happens on start up of my map in Hammer.
To fix: This is only temporary but it works just fine. Face your camera AWAY from the water brushes, and switch the grids from Lightmap -> 3d shaded textures. This should remove the phantom brush.