Yes, it does make the water-tight surace. Maybe the problem was that the trimming curve was too close to the edge of the surface?
Bad NURBS surfaces are as common as bad meshes. Rhino has many commands which repair bad meshes, so it is natural to expect equal number of Rhino commands that repair bad NURBS surfaces. In reality, there are few if any such commands. The Check command is nearly useless - it calls bad surfaces good surfaces. Worse yet, there are no diagnostic tools to help us figure out what is wrong with a bad NURBS surface.