Thanks Torq and Ascension - I love doing this and the encouragement and advice I've picked up over the last year are the reason I'm still doing this.

@Nomadic: The joy of it is that you already can. The pool in the first post screenshot was created using the following steps:

1. Use the tilable flagstone texture to lay down a floor.
2. Use the Step tiles (in the stairs and elevation subdirectory) to define the edges of the pool.
3. Use the water texture to paint in the pool water.

If you are using maptool, just make sure that you paint all the textures on the background layer. Then when you place the tiles on the bakground layer they will define nice sharp edges.