In order to be used for Advanced Hydrostatics, the model must be one or more closed surfaces or polysurfaces. This means no open ("naked") edges or points. If the model is not closed, an error will be shown when creating a new Design.
To check if a model is closed, select the model and look at the Rhino Properties for the Object:
If it is not closed, you may find the Rhino showedges command to be helpful in finding the naked edges or points. This Rhino FAQ has some strategies to close naked edges, including the use of the showedges command and a discussion of tolerances: https://wiki.mcneel.com/rhino/faqclosedsolids
The Absolute Tolerance that should be set at the beginning of your modeling work is important. If the Absolute Tolerance is too small, it can be difficult to create a fully closed polysurface. We often see models where the units are mm or inches and the Absolute Tolerance is set to 0.001, which is too small. A value of 0.1 mm or 0.01 inches is more reasonable. This Rhino FAQ is helpful in understanding tolerances: https://wiki.mcneel.com/rhino/faqtolerances
If you are having difficulties getting rid of naked edges and you find that your tolerance is too small, you may need to explode and rejoin the model with a larger tolerance. In some cases you may also need to untrim and retrim after adjusting the tolerance setting.
You may find the command JoinEdges useful to join naked edges, and the command RemoveAllNakedMicroEdges to remove naked points.
It is also important that the outward normal direction of your model, shown with Rhino's dir command, is pointing out of the model. If not, you can use the flip command to correct it.