Swimmable Layers
This pictorial attempts to explain the theory of swimmable layers and also shows what constructions are required for a swimmable pond/lake.
A standard swimming pool ...
... is a hole in the ground, with a diving board and a set of steps. But what's going on here?
If we call the ground "layer 0", the question is "what layer are the sims swimming in?"
If they are in layer 0, it would be possible to place a diving board on the ground and have them dive from it, into the ground. But they can't. So they must be in layer -1.
To test this, we'll raise the diving board onto a foundation (the top of which is layer 1) and see if they can "swim" in the ground (layer 0). If they can, we should be able to use the diving board and pool steps between any layers that are one apart.
Well what do you know!
(You'll need to use the "moveobjects on" cheat to position the board and steps, but other than that, no "magic" is needed.)
So why can't sims swim in a pond?
As far as I can tell, the surface of the pond is NOT a layer - it acts more like a roof. A roof is a convex surface covering a flat layer, while the water surface of a pond is a flat surface covering a concave layer.
So, how can we create a swimmable pond/lake?
Well, the tricky answer is "we can't". What we actually create is a swimmable floor that has been sunk just below the surface of the pond/lake.
This is what we need to build, where the blue carpet is in the lake and the steps are on land.
As layers at different heights cannot touch, we have to use stairs to get from the land portion to the water portion.
With "boolprop constrainfloorelevation false" set, use the water tool and slowly lower the four corners of the tile the column is on until the supported carpet tile is just below water level.
Note: To place the upper decking, hold down the shift key while dragging to the required size.
Using the water tool, make a shallow pond the same size as the swimmable floor (the blue carpet), don't worry if the bottom is not level, we'll fix that next. What is important is that there is a pond under all of the swimmable floor.
Now, using the levelling tool, level from the bottom of the fully sunken pillar across all the pond. If you didn't use the water tool properly your water will vanish at this stage! If this happens, just undo and expand the pool.
Bulldoze the steps, use CFE techniques to lower the land structures, and replace the stairs. Delete the board and steps and place new ones using "moveobjects on".
Remove the supporting columns, change the swimmable layer to a transparent blue tile, soften the edges of the pond/lake, add water plants to hide the edges, decorate, etc, etc, etc.
Note: The solid foundation can be deleted with the sledgehammer tool if it's not required. The piles on the upper deck will extend to the ground, but the "cans" will remain where they are. They can be hidden with bushes, etc. As the upper decking is layer 2, sims will also think they can walk under it, so you may also need to fence it off.