Interest in the study of next-generation underwater sensor networks for ocean investigation has increased owing to developing concerns over its utilization in areas, such as oceanography, commercial operations in maritime areas, and military surveillance. The underwater base station controller, in the form of a surface buoy, communicates with underwater base stations (UBSs) while the UBSs transceiver information with underwater sensor nodes via acoustic communications. This paper provides a link-level and system-level study of downlinks using an orthogonal frequency-division multiple access techniques for underwater acoustic communications (UAC) networks. We present an approach to link-level-to-system-level (L2S) mapping at the link level that provides an abstraction model of the link-level performance to be accessed by system-level simulation (SLS). In this paper, an exponential effective signal-to-noise ratio (SNR) mapping (EESM) method is adopted, which elaborates on how multi-state channels are integrated into a single state in an SLS, and why an effective SNR can represent the characteristics of a multiple subcarrier SNR. Moreover, we explain the beta calibration procedure in detail for a UAC network. The simulation results are provided to verify the beta calibration of the UAC network. Furthermore, we employ a link adaptation strategy by evaluating system throughput based on a proportional fair (PF) scheduler at the system level. Hence, the simulation results confirm the effectiveness of link adaptation strategy for UAC networks.