Water brushes.

To make a water brush, use a texture that has a astrik(*)(sp?) as the first
character. *WATER0 is the basic water texture in Quake. Also, water brushes
can sometimes be confusing. With a solid brush, you define the area where
you can not go. The water brush is open space, as long as a solid brush
does not cross into it. See WBOX.MPB for an example.
-Dave
