I have found that a good strategy in creating a watertight volume from surfaces is to try and make the surfaces extend beyond adjacent surfaces. It looks like you expect that all your surfaces are appropriately trimmed. They are not. For example, look at the area I have highlighted below. The red surface is a smooth surface in 3D whereas the green and

Changing to wireframe visual style and zooming very tightly to the boundary between the red and cyan surfaces you can see a discrepancy.

I recommend extending the red dome if possible and the bounding surfaces around it.
Another thing you could do is to examine parts of your model using planes to define the boundary of the watertight volume. For example, in the image below I created 4 planes then gave the surfsculpt command and selected a few of the top surfaces of your model that covered the area within the planes, the 4 planes and the bottom surface of the model.

The result was the solid you see below.

Try this on a larger portion of the model.
A side note, I note that you have several layers turned off. I NEVER use layer off but much prefer frozen instead. A frozen layer is more off than an off layer! E.g., select all will select objects on an off layers but not on frozen layers.
lee.minardi