This paper presents the architecture design and the performance evaluation of a composite-based back end and visualization systems. The first one is used to host composites representing sensor nodes, store, index and manage water quality measurements collected by wireless sensor networks (WSNs) deployed in rivers, lakes and coastal regions. Embedded communication systems in each sensor node enable ad hoc network operations to relay the measurements to its composite in charge of managing, processing and providing data to end users and large communities through a standard web RESTful API. The visualization system is in charge of providing web interfaces to scientifics and users to monitor and configure the wireless sensor network (WSN) and visualize measurements in a real-time manner. The focus is on both the back end and the visualization systems architecture description and on the evaluation of their performance