I always feel thirsty too (well sticky mouth anyway). I have done 3 wks on 3 lts, and now trying to drink 4 lts. I think is more the feeling in my month that makes me feel thirsty, try brushing your teeth more?
Dont forget that you may not have drunk much water before starting CD, but 'normal food' contains a high % of water, plus cups of tea/coffee/diet coke and all the G&Ts too! You would be suprised how much liquid you are missing out that you didnt realise you used to have.
xx