There is a growing interest for underwater sensor networks where long term monitoring of water masses around the world for scientific, environmental, commercial, and military reasons is desired. In this paper we will present the concept of a highly flexible acoustic modem called the Reconfigurable Modem (rModem) that can be used for rapid testing and development of such networks. Categories and Subject Descriptors B.m [Hardware]: Miscellaneous General Terms Experimentation Keywords underwater, acoustic, network, experiment, rapid prototyping, rmodem.