I've had real viscious headaches in the past, though not when diving, and many of them can be attributed to dehydration. Something which now ensures I often drink enough before a dive that I need to be poured into my suit!
If I get a headache, as well as a couple of ibuprofen or paracetamol I also try to drink at least a pint of water, or more if I can.
A quick look on google for "diving headache" came up with the following.
(I didn't write any of this, I've only cut & pasted it - I make NO claims as regards it's medical accuracy. I am NOT a doctor and make no claims as such. If you have problems then I suggest you speak to a diving doc - there, that should stop anyone trying to sue me)
http://www.peebsac.ukdiver.com/peebsac_headaches.htm
and
http://www.scuba-doc.com/headache.html
and from another site -
Quote -
DIVER'S HEADACHE:
Diver's headache can have several causes. These include neck muscle headache, hyperventilation vascular headache, sinus headache, fume headache, tension headache, dehydration headache, hypertensive headache, and (worst case) decompression illness. The best treatment for any of these headaches is prevention. If you commonly get a headache while diving, try to identify the cause, then eliminate it.
Neck muscle headache: This headache usually starts at the back of the head, but can become generalized. It usually begins gradually after a couple of dives. It's caused by chronically tilting the head up during the dive -- for example, looking upward while swimming horizontally. The muscles where the back of the head meets the neck may be tender to touch. Ibuprofen (up to 800 mg every 6 to 8 hours) and ice packs can help.
Hyperventilation vascular headache: This is a pounding headache that can occur at any time during or after a dive. The cause is breathing more rapidly that you should (hyperventilation). So it's more common in free-divers than scuba divers. Reduced carbon dioxide levels during hyperventilation lead to constriction of the blood vessels of the brain. When the vessels relax again, they often dilate and throbbing pain begins. Sensitivity to light and nausea are common. Caffeine is often very helpful for this type of headache, when combined with ibuprofen, aspirin, or acetaminophen.
Sinus headache: This headache begins with congested sinus openings, so the sinuses don't equalize during descent or ascent. The pain is usually most intense in the face area, and often begins as an ache in the cheekbone, eye, or forehead. Besides ibuprofen (800 mg), use a nasal decongestant spray (example Afrin) to unplug the sinus. If blood and pus-like material come out your nose, you have progressed from sinus congestion to sinusitis -- and you will need antibiotics.
Fume headache: Diesel fumes. You need clean air.
Tension headache: "Ordinary" generalized pressure-type headache due to travel hassles, disrupted sleep patterns, and altered activity patterns. Treat with rest, fluids, and an analgesic such as ibuprofen or acetaminophen.
Dehydration headache: This headache throbs when you stand up. It seems to be better when you lie down. Often, you may have a "head rush" when you first stand up. The headache occurs because decreased blood vessel volume lets the brain "sag" down when you're upright. Alcohol, caffeine, and snug wet suits make this headache more likely. (Alcohol and caffeine act as diuretics, flushing extra water out through the kidney. Wet suits compress the veins of the skin, pushing blood into the central circulation. This fools the body into thinking there's too much fluid in circulation, so it orders the kidney to eliminate some -- which causes the familiar need to pee in the wetsuit.) Treatment is rest and lots of fluids.
Hypertensive headache: Some divers are sensitive to decongestant medicines such as pseudoephedrine (Sudafed) and phenylpropranolamine. The blood pressure can jump up significantly. Combine struggling with heavy tanks and cumbersome wet suits, and a hypertensive headache develops. This headache often comes on during the physical activity, and eases once you slow down. Have your blood pressure checked during the headache. If it's elevated, stop activity until it goes away. Avoid decongestants in the future.
Decompression headache: Any new headache after diving accompanied by a neurological symptom (blind spot, localized weakness, numbness or tingling, etc) should be considered a symptom of decompression illness until proven otherwise. You need immediate medical attention -- and probably a compression chamber.
Unquote
Hope that helps in some way.
Dave