This paper describes the design process of a modular system for supporting physical and cognitive rehabilitation in the swimming pool. In such an environment, the therapist is called to creatively adapt rehabilitation protocols to the enhanced ability of the patients, often reacting to emerging behaviours enabled by the water. Therefore a strong technological requirement for such environment is to develop a modular system that can be configured and modified “on the fly” during the activity, exploiting the therapeutic properties of the water. To satisfy such a requirement the system of Active Surfaces has been developed. It consists of a number of position aware floating units, called tiles, able to communicate each other and to provide visual, acoustic and tactile feedback. By combining the different tiles the therapist can easily configure the dedicated tasks for the various typology of patients. The concept has been developed following the Palpable Computing approach, an innovat...