Instructions
Load a lot with a Sim and a house already saved. If you have not yet created a Sim and a house, select a house that has been prepopulated or create a Sim and move her into a vacant premade house.
Press "Ctrl," "Shift," and "c" simultaneously on your keyboard to bring up a text field in which players can enter cheats.
Type "move_objects on" exactly as it appears in the cheat text field, then press "Enter" to activate the cheat.
Select the "Build Mode" icon from the interface, then choose the "Pool" tool from the menu to the right of the "Build Mode" icon.
Press "Lower level" until you are below the lot, editing the basement.
Apply the "Pool Dropper" tool to each square of the "Build Mode" grid that you want to be a pool. Add at least one ladder from the same menu so that your Sims can climb in and out.