Rheological properties and water activity of aqueous dispersions of Laponite have been studied in order to define water implications for structural and mechanical properties of soft colloidal materials. Different experimental parameters, such as Laponite concentration, time of rest and temperature were analysed. It was shown that solid-like rheological parameters increased with the concentration, the time of rest and the temperature while water activity decreased. More interestingly, it was highlighted that storage modulus and the inverse of water activity are linearly connected regardless the experimental conditions. Water activity finally appeared as an easy measurement to evaluate the structural water bounding of Laponite dispersions.