I have this situation all the time. Trying to compute the volume of water inside the spas I design.

I have attached a very simple one. Most of the spas I design have many more internal surfaces and opennings.

I have played around with creating offset surfaces by selecting faces, but then have problem when surfaces have round holes through them. Typically the water level is 6 inches below the top surface of the spa.

Thanks for your suggestions. I use SW2011

Site wouldn't let me insert an image so I also attached jpg image of spa.

UPDATE: For some reason I can't attach any files. Generates some type of site error. Which makes this discussion tough cause you can't see what I'm talking about!

Trying to descibe the spa better, there are internal steps, some with large radius edges. Benches are at different heights. Sometime floor has a section that goes much deeper. Any not all of these bodies are merged. So I try to do offset surfaces, selecting each surface. But then I have to create the water surface. Then want to merge all of these surfaces into a solid. Up till now I can't get this idea to work.