People who are completely healthy and go to high elevation without any symptoms can drink a lot of water without a problem, and it just keeps the kidneys working. People who are taking high doses of Tylenol and things like that need to drink a lot of water to avoid a kidney problem. They rest of us just drink as much as we need, and experience has taught us about how much that is. If I don't urinate at least twice per day, then I am probably dehydrated.
I understand that you report headaches, but it might be from the simpler forms of altitude illness. Full-blown HACE is kind of uncommon except at very high elevation (like 18,000 feet and above). The only person that I saw with HACE symptoms had been to nearly 23,000 feet. However, just last summer one friend got sick around 10,000 feet, and it was thought to be some combination of HAPE and HACE, or at least the symptoms were a combination.
Yes, if you do take Diamox, you need to start it a day or two before you start going high. Exactly what elevation that is will vary from person to person. A bunch of us were going up very high (nearly 23,000 feet), so we started a half-dose of Diamox the day before we started from a 9000 foot trailhead. Then we went to a full dose when we went above 14,000 feet.
Last year when I was descending from a 14,000 foot peak, I saw a large military group starting up from 12,000 feet. Apparently some were taking Diamox, some were taking some new secret drug, and probably some were taking a placebo. I would love to see the results from that test. The military people were headed to Afghanistan.