Ok, first off those are obvously not the default terrain tiles.

The large airfield underlying terrain tile is this...
afgrass5.bmp

It takes up the FULL 1 mile terrain square. NOT the 1/2 mile quadrant.
So, in order to make an airfield that has the water as you show it. You would need to make a single bitmap out of the 4 quadrant tiles AND create a new airfield object with AC3D (I think) and import it into the TE.
You would need to do this for each different terrain layout, or accept the fact that the terrain on all airfields will look the same.
Again, I'd sacrifice the water and just put a regular airfield in there. The trees bushes and hills are ground clutter and again you'd need to create new ones with AC3D. Even then, since ground clutter is pretty much random you would still wind up with those things in the middle of your water.
Remember, just because you have water displayed in a texture, it doesn't make it water.
