In one study, 14 participants drank 16.9 ounces (0.5 litres) of water. Drinking water was found to increase their metabolic rate by 30% for 30–40 minutes.
Another study looked at the effects of drinking water in 21 overweight children. Drinking 0.3 ounces of water per 2.2 pounds of body weight (10 ml/kg) increased metabolism by an impressive 25% for 40 minutes.
Research on lemon water specifically is limited. However, because water is the main ingredient, it likely carries the same metabolism-boosting benefits as regular water.