We used two 750ml bottles and that was plenty - you need to make sure you drink a lot, especially in hot weather. An "average" human should consume 2-3 litres a day - when exerting make that 4-5 - in hot weather make that 6 or more. Some of that should be rehydration drinks (even something like isostar) to replace salts but do NOT count sweet drinks like Coke.
The early signs of dehydration are not things like thirst it's things like losing energy, becoming very emotional, struggling with usually simple actions etc... and it's just irresponsible to ignore this.
Coke and similar drinks require your body to use water to process the sugar content so although they are liquids you dont really get a net fluid gain from them. However, if you're suffering the symptoms of dehydration or sunstroke a cold coke - or much better still a cold, flat lemonade - is a life saver (possibly literally!)
Contrary to popular belief coffee does give you a net fluid gain but too much will cause other problems....like realising how crap the coffee back home is

"The journey of a thousand miles starts with..." Discuss.