Near the pool, create stairs 1 step down



Delete the stairs and level the terrain. Then create a small pool.



I've colored the pools to explain how we want to level it.
BLUE are the normal pool floor levels.
GREEN are the 1-stairs down pool floor levels, using the the small pool as reference
WHITE are the slopes
Use the level tool, click on the small pool and drag it to the main pool to level it.



The pool should end up like this, viewed from the pool window's room



EDIT: I've made the above shape so the effects will be visible. All you really need is the GREEN level next to the pool windows, followed by the WHITE slope and then the rectangle BLUE levels. Like the following:



Notice the edge of the pool floor connected to the window is level, the water level is still correct


EDIT: When your done, don't delete the small pool yet. Constrainfloorelevation TRUE. Zoom out to the town, zoom back in your lot, THEN delete the small pool.

EDIT: (Credits to jrclark for discovering this)
This method only works for 2 sides of a pool of any lot. Please read post #18 - Solution Found by jrclark

This is the end result. Sims can "swim here" anywhere in the pool. The water level and the pool floor levels remains after reload or moving in/out, or even placing items in the pool.


As an added bonus for using this method, your sims get to swim in the BLUE area and slowly submerges as they approach the pool window's GREEN area.

This method is also good for 2-sided pool windows like this.


You can have more windows. Just remember this:

GREEN levels for all floor tiles NEXT TO THE POOL WINDOW
BLUE levels for tiles WHERE SIMS CAN GET IN/OUT OF THE POOL

Never allow your sims to get in/out of the pool via the WHITE or GREEN
levels! You'll have glitches in animation (whether dive in / climb out, or using the pool ladder)

Make sure the BLUE areas are the only place your sims can get in/out of the pool. Block entering/exiting the pool via the WHITE and GREEN areas with walls, foundations, ladders, plants, or other objects.