what to drink during longer practice

Hi, I notice that I tend to get headaches after intense, longer swim practices, even though I feel fine during the practice. I drink plenty of water before and during practice but still get the headaches. I also noticed that I tend not to get the headaches when I have some sports drink mixed with water during the swim. So I'm thinking the headache is connected with a drop in blood sugar. I have been using Gatorade, but I don't like that it is so high in sugar, so I'd like to find a better option -- either something I could buy or make myself. Any suggestions? Here are my criteria: Low in sugar but not with artificial sweetener, including xylitol (which gives me digestive issues) Not loaded with excessive vitamins or strange ingredients Not expensive No caffeine Any suggestions would be greatly appreciated! Thank you!