I too get headaches when Im fishing, tried all sorts, thought I was just dehydrated but drank loads of water on next trip and still have the head aches, cant work it out, could it be the sunlight reflecting of the waters surface? Never packed up and gone home though! just monster trucked on and still blanked. Maybes its just the thought of trying to find a nice spot in a very weedy lake that give me the head aches!
With regards to taking something - A travel hammock! Best thing I took! when resting me swims and watching the lake, set up under a shaded tree swinging around! cant beat it!
I get frequently get headaches when I'm fishing abroad- but its typically dehydration. I spend all of my time spodding, casting, tying rigs and generally sweating- without drinking any water. I always have 1 or 2 dyrolites a day and that helps. Co-codamol seems to work better for my headaches than paracetamol.
Top advice from Fenboy too on anti histamine tablets. They can be a life saver if you do get hammered by mozzie's.