Problems with Geometry
Check for Watertightness Tools insists that an Edge is not watertight
A common problem that occurs when trying to correct geometry is that the Check for Watertightness Tools appear to be overeager: they insist that an edge is non-watertight, even though it is not.
Actually, the Check for Watertightness Tools are never wrong. What is far more likely is that the geometry is arranged such that the problem is somehow impossible to see (for example, a razor-thin overlap) – but it is nonetheless there.
Common correction-related problems are covered in the section on Correcting Geometry, especially the section on Common Problems. You might find an answer to your problem there.