Anyway, I re-shot the pictures and here we are. One tutorial, coming up :)
I've chosen the Takemizu Hot Springs as base for this tutorial, because this was the building that forced me to learn how. So here it is in all it's eaxis glory.
For your pool, remove the foundation where you want your pool to go.
Make an outline where you want the foundation to go when you're done.
Next, using the medium terrain raise tool, raise the terrain four mouse clicks.
Next, raise the terrain in the entire pool area. As you know, without the use of cheats, it is impossible to raise covered terrain. That's fine, just raise the maximum amount of interior terrain that you can for now.
Place a pool on the raised terrain.
Next, since we do want the pool to cover the entire area in the middle, turn on this cheat:
boolProp ConstrainFloorElevation false
Fill in the pool area that's still grass.
You can turn off the FloorElevation cheat now. Just type:
boolProp ConstrainFloorElevation true
Place foundation around the pool. You'll have to do this one side at a time.
And there it is, a pool inside a foundation. Sims will dive into anything, so use the move_objects on cheat to place a diving board or two and a couple of ladders out of the pool. Keep in mind that sims will look like they're swimming underwater, because of how the pool was build. Other than that, it works like any other pool :)
Good luck building your pool anon!