I know this happened to me recently, actually on my last dive. I went down 87 feet (deepest i have gone to date) and when I came back up, i realized that I had a headache.. I figured it was lack of hydration in the AM before my dives. I usually drink a lot of water the night before and the morning of the dive, but for whatever reason, last dive, no water. Also, because it was the deepest i have gone, I think i was a wee bit anxious, and was breathing heavier, non relaxed so that also did not help at all. ....