This is the best £25 I ever spent on a sack...



The main compartment is totally waterproof, it has a huge storage capacity for its size (7 liters), and will expand (note the roll top). It can hold two water bottles. It is so comfortable I forget I have it on. There is also a good deal on at the moment.

http://www.craghoppers.com/Craghoppe...es/CHT1017.htm