I definately notice the difference with being well hydrated. I'd say 2 things though:
1st - it takes a while to get used to drinking 2 litres a day, you pee a lot to start with
2nd - don't drink lots of water in one go it doesn't help hydration, you just wee it out, sip steadily through the day.
Drinking out of streams is great, just watch out for pollution and dead sheep. Was doing the 3 peaks a couple of months ago and spotted a dead sheep in the steam (hidden behind a big rock - only spotted it cause it stunk). Yeuch!!

