I have a client who wanted a simple pool and I decided to see if I could model in chief.   At first I scratched my head a little, but then I figured out it could be done without much issue.   I created a terrain perimeter just larger than the pool.   I drew elevation lines to make it slope from 3'6 to 6' along this path.   I then drew the simple kidney shape (but it could be any shape).   I found a good reference point at one of the tangent points that I could use when "layering" up objects.