There are currently no digital underwater communications standards. In this paper we describe JANUS, a simple multiple-access acoustic protocol designed and tested by the NATO Centre for Maritime Research and Experimentation (CMRE) over the past 6 years that provides a basic and robust tool for collaborative underwater communications. JANUS is in process to become a NATO standard but is not intended to be solely military nor only for NATO, but also for civil and international adoption. JANUS is unique in its open and public nature such that academia, industry and governments may all benefit from its use. The specification of the signal encoding and message format is fully described so that anyone may construct a transmitter/receiver to communicate via JANUS to any other compliant platform. While JANUS is deliberately simple to allow easy adoption by legacy equipment, the protocol also offers the freedom to utilize sophisticated receivers and decoders allowing performance to be significantly improved.