What I do is create a sketch at the water level and extrude to body, turn off merge results,
then you can check the volume of the water. if you have holes on the bottom you might have to fill them in.
I would use the method described by Frank, but apparently SW13 has an Intersect function which sounds like it will do what you need.
Dale, as Kelvin said, the Intersect tool would work like a charm here.
If the spa is a surface body, I would extrude a rectangular body of water larger than the spa and trim with surface keeping the water. If the spa is a solid or thickened surface I would use the subtract form of combine and keep the water. Another approach is to project the water line to the surface of the spa, then in the plane of the water insert entity for the edge of the water surface, finally extrude to surface of the inside of the spa. There are many more methods. It might be fun to do a rippled water surface, search this forum.