Races have water and food stops along the way normally. So, while many people train with water, most just rely on the aid stations for the race. I wear a pair of race-ready brand shorts for marathons. They have tons of pockets for storing my vaseline, GU, and glocose tablets. That way, I only have to worry about fluids. I do train using the same sports drink that the particular marathon says they're going to use. This is very helpful - due to the yuk factor of some! I've never run a race carrying any liquids and I've been fine. However, I'm debating carrying one 16oz bottle of water for the start of my next marathon. But my needs are different, I'm a breastfeeding marathoner and I find I need much more water than the average person. One 16oz bottle isn't much, but I figure it's better to be very well hydrated early on and it shouldn't be a hot race.