Some divers bite too hard on their mouthpiece or have their mask-strap too tight, which can have the same effect. Anxiety on stressful dives can cause a tension headache. Carbon dioxide retention, which might be the result of problems with excessive equipment deadspace or inadequate ventilation of the lungs during dives, is another cause. Sometimes CO2 retention is the result of a deliberate attempt by a diver to reduce air consumption. Cold water can cause a headache identical to that which is triggered by eating something cold.
Further causes are sinus barotrauma and decompression illness.
We now recognise that some types of migraine after dives are particularly related to decompression illness. Migraine aura takes many forms, but the most common is visual disturbance, with zigzag lines or flashing lights either before, at the same time as or instead of the headache. Amputated fingers I lost the tops off two fingers in an accident at work. The doctors say that the wounds should heal up soon. I do not anticipate any problems.