Conventional wireless sensor networks (WSNs) are increasing in use; however, not all environments can support the typical wireless sensor model, which utilizes RF as a communication medium. Hostile sensing and communication environments, such as underwater and subterranean, require the practical use of other methods. In particular, the underwater domain has necessitated the use of acoustics to enable communication in underwater WSNs. Previous work in this area consists of sensor nodes built with bulky, costly acoustic modems. In contrast, we present the low cost, small form factor CORAL testbed, which consists of piezo-transducers, a microcontroller-based architecture and interface circuitry. We illustrate the operation of a prototype, show the performance in various environments, and explain its integration into a micronode for underwater WSNs. We conclude with a discussion on the results, limitations, and challenges, as well as a discussion on potential applications.