This seems too good to be true but there is a study that provides some evidence for this claim.
Metabolic rate was measured in healthy, normal-weight men and women after drinking 500 ml of water. Results
showed that metabolic rate increased transiently by 30%. There was an interesting gender effect showing the
increased caloric expenditure came from fat in men and carbohydrate in women. The researchers believe the
increase in metabolic rate was partially due to water-induced stimulation of the sympathetic nervous system,
because when they administered a drug that blocks B-adrenergic receptors, the increase in metabolic rate was
prevented. Before you go out and drink 2 gallons of water trying to lose those love handles, let's put this
into perspective. If 500 ml (about 2 cups) of water was ingested three times per day, this would translate
into an additional 50 kcal burned per day. If this were done every day (assuming no compensation over time),
it would take 70 days to lose one pound of fat.
Boschmann M, Steiniger J, Hille U, Tank J, Adams F, Sharma AM, Klaus S, Luft FC, Jordan J. Water-induced thermogenesis. J Clin Endocrinol Metab. 2003 Dec;88(12):6015-9.