For a full day walk, you really need about 1.5 litres of water in cool weather, up to about 2.5 or even 3 litres during the heat of summer. It’s a good idea to drink lots (but not coffee) before you start walking, to give you a flying start! Personally we don’t see any need to purchase brand name water bottles – it’s perfectly OK to use discarded soft drink bottles which are lightweight, plentiful, durable, don’t leak and they are free!
Many people like camel baks – a “built in” water sack that sits snugly in your day pack and has a tube from which you can drink. If using one of these hydration systems, it is good practice to have a spare bottle of water in your pack, just in case you suck the camel bak dry!