I'm not telling people to all go shave their hair off, I'm just saying that's what I did. Tying your hair up isn't really going to stop your head getting hot under a helmet, when your hair is insanely thick like mine. You might have less hair on your neck, but all the hair is still on your head, between helmet and skin, and being a horrible, sweat catching, insulating wad of gross. I tried everything I could think of before actually shaving it off, but I never regretted doing it.
With the water, I was told that water can damage helmets, but I used to do that a bit. Works for the first half hour, then you spend the next three hours with gross, damp, warm hair and an even stickier helmet than what sweat alone causes. Hell, when I was a kid I'd ride out to get the cows in and have a dunk in the trough before following them back. Didn't have a helmet then though. XD