Final answer:
The priority assessment for the nurse is to ensure that the tubing is free of any occlusions.
Step-by-step explanation:
The priority assessment that the nurse needs to make when the water is below the required level in the water seal chamber of the closed chest drainage system is to ensure that the tubing is free of any occlusions. An occlusion in the tubing can prevent proper drainage and may cause complications. Checking for bloody drainage and evaluating the client for the presence of a pneumothorax are important assessments as well, but ensuring that the tubing is free of occlusions takes priority.