Hydrostatic won't allow surface selection

I have a modeled geometry that I would like to run the Hydrostatics command on, in order to return information like Volume Displacement or Center of Buoyancy. The Hydrostatics command requires the user to select the surface of the body that the calculations will be performed on.
However when I Select > Surfaces, Rhino says 'Cannot use these objects. Removing them from the selection.'
What should I try in order to get the Hydrostatics command to work?

Here is the solution. I had to use Join to join the surfaces to define one object. I think CreateSolid could do this too.