Abstract: A method of using liner-like cores or core assemblies for creating in-ground concrete spas or swim pools. For larger projects, that is for enlarged scale spas or swim pools, the inventive liner-like core comprises an assembly of sections. The cores or core assemblies are re-usable at least for several projects if reasonably maintained and cleaned. The core assembly comprises two long walls and two short wall defining the perimeter around a split level bottom with a shallow end, a deep end, and an intermediate transition slope. The liner-like core is constructed from a suitable rigid material like fiberglass for stiffening purposes and is an assembly of sections and also comprises: leveling blocks, transverse support beams, and hangers that attach to the walls of the core assembly and stiffens the long walls against being warped inward during the concrete pour.