I've had a lot of requests for curved pool tutorials so I made some diagrams to show the exact sections needed for the following pool shapes. I hope this helps. If you want me to diagram a specific shape for you, send me a picture.
Make sure the area of the pool is flat.
Lay down a floor tile over the area before you start. You can always remove it later if you need to, and it will help to stop curved tiles from changing shape to square.
You may need to use the "constrainfloorelevation false" tool in the console(ctrl/shift+c) if the game won't let you place pool sections. Don't forget to switch back to "constrainfloorelevation true" afterwards.
You may need to use the console command "moveobjects on" to place your ladders and lights.
After you make the sections on the diagram, fill in with regular square pool sections.