Hi Nick, yes that is correct.

When it says "Solid", that means that every edge in that object is joined to another edge. So yes you could call that "watertight".

If it does not say "Solid", then it means that at least one edge in the shape is unattached - I mean it is not joined to another edge. So the skin of that object is not fully sealed in that case - not watertight.

If an object like that is made up of more than one surface (where there are some joined edges between them, just not all edges joined), then it will show a label of "Joined srf" for that, and if it is just one surface all on its own (like a plane for example) it will say "Surface".

- Michael