We present a lake testbed that can support mobile acoustic communications and networking research. The primary assets of the testbed are two autonomous underwater vehicles equipped with software-defined acoustic modems. The acoustic modems are integrated with the user-installed autonomy software on the vehicle, MOOS-IvP. Through this integration, vehicle information can be passed to the acoustic modem. Or the acoustic modem can receive remote instructions to modify vehicle missions. With additional stationary nodes, the testbed can support two types of acoustic experiments. First, it supports acoustic waveform transmissions from two synchronized mobile nodes. The acoustic transmissions can be recorded in a multi-element receiving array for offline data processing. Second, the testbed can perform real-time acoustic communication and networking tests for different vehicle trajectories. We will present the testbed instrument and capabilities, field deployments, sample acoustic data, and communication test results. [The research is supported by the national science foundation (NSF).]