My thoughts on this:
I prefer sports drinks to water, but that is a choice - for some folks water is fine, but I am not one of them.
Be-aware that sports drinks (like cars) come in different types - often the best known are no where near the best ones to use. There are 'sports' drinks that are only "sports" because the marketing dept said so - these are pretty much sugar water (Gatorade and Powerade are close to this) and not much else - they are also rather expensive to buy.
The drink of choice for me is Endura. Ifind it really helps stop cramping and assists with re-hydration. We are all different so you milage may vary, but I would say purchase Endura over gatorade/powerade anyday. It is far better to bulk purchase it as well. 800 grams of powder is about $26 dollars - I recommend you purchase thru Vitiman Me. either online or via their shops as they usually have the best price. I have no professional or commerical interest in either of these recommendation, they are just what I have found works best for me and where the best values for money and performace are.
Regarding your headaches etc. In addition to drinking enough fluids, make sure you move your neck and head around when riding longer distances. You want to have a little strech and wriggle occasionally, otherwise you can may stay tensed up - this can cause headaches, neck aches and other issues. Try and relax into the ride and move around a bit to keep the back, neck, arms and wrists flexible.