this is from http://www.low-carbdiet.co.uk/

Water is essential to weight loss for those who eat low-carb. The minimum consumed in a day should be

Weight-----------------Liters
140lbs-----------------2.5
160lbs-----------------3.0
180lbs-----------------3.0
200lbs-----------------3.5
220lbs-----------------3.5
240lbs-----------------4.0
260lbs-----------------4.5
280lbs-----------------4.5
300lbs-----------------5.0
320lbs-----------------5.5
340lbs-----------------5.5
360lbs-----------------6.0
380lbs-----------------6.5
400lbs-----------------6.5

High levels of ketones in the blood stream can lead to a reduction in ketone production, therefore being well hydrated could aid in keeping the levels low and ketone production ongoing. Consuming enough water can have many other positive side effects: aids your kidneys with the processing of protein, reduces the retention of water, helps with preventing constipation, and reduces the levels of ketones released by your breath, which in-turn will reduce breath odor.