CAD tip # 13163:

You can use 2 or more 2D profiles of any shape to create a 3D surface or closed (watertight) solid. The command creates a transition shape between the specified profiles. A profile can be virtually anything - a 2D polyline, 2D solid, 2D spline, arc, circle, an edge or face of a 3D solid, ellipse, spiral, line, surface, region, trace or even a point (only the first or last profile). You can also specify the normal (method of lofting) on each profile. If the LOFTNORMALS variable is set to 1 (smooth fit), you can use the arrow nodes on each profile to specify the continuity (G0, G1, G2) and bulge of the shape transition for that profile. The special mode "Draft angle" (see the main options node) allows you to specify the draft angle of the transition shape on the specified profile:

You can then create a shell (e.g. a pool body) from the resulting solid. Just specify (by picking with Ctrl) the area that will be removed from the original solid - before the shell walls are thickened - i.e. where the solid will be "opened". If you want to open the shell in more than one place, select more faces. Then you just need to specify the shell thickness.