In normal weather for 40 miles, water should be fine. If it's real hot, you go for longer distances or you have problems with cramps you might want to try additives that fuel you and replenish electrolytes.

I usually use regular water and add some Coke or Mt. Dew after 20-30 miles. I'll stop at a 7-11, get a 32 oz drink, drink some of it and pour the remainder in my bottles.