The management of natural resources is an intricate and consequential task. In particular, water management is at the forefront of public policy priorities in many countries because of its growing scarcity and its considerable economic and social implications. At the core of water policy is the need to foster a more rational use of the resource, and one way of fostering efficiency might be the creation of an agile market of water rights. However, the design and operation of such a market is not an easy endeavor because it needs to coexist in a complex social and legal framework that has evolved to address the different and often conflicting objectives of the many stakeholders involved. We are approaching this problem by building an open multi-agent system, mWater, that is designed as a regulated environment where autonomous agents trade rights for the use of water in a closed basin. mWater is intended as a sophisticated simulator of the demand component of a basin for the design and testing of water management policies, as a test case for a potential actual market and as a sandbox for the development of agreement technologies. This paper outlines the aims of the mWater system, describes its core institutional components and indicates its potential use for the development and testing of technologies involved in the processes of reaching and fulfilling agreements.