Final answer:
The highest priority nursing diagnosis for a client with a cervical spine injury is 'Ineffective breathing pattern' due to the potential for respiratory compromise.
Step-by-step explanation:
When planning care for a client with a cervical spine injury sustained during a diving accident, the nurse should assign the highest priority to the nursing diagnosis Ineffective breathing pattern. This priority is due to the risk of respiratory compromise, which can be life-threatening. Therefore, maintaining an open airway and ensuring adequate ventilation and oxygenation take precedence over other nursing diagnoses such as 'Dressing or grooming self-care deficit', 'Impaired physical mobility', and 'Disturbed sensory perception (tactile)'.