The past few years have witnessed a significant progress in the research of underwater wireless networks. Many protocols and applications have been proposed. However, due to high costs of field tests, not all research work can be evaluated in real environment. To facilitate the researches of underwater networks, we develop a lab testbed based on the Orthogonal Frequency Division Multiplexing (OFDM) lab modems and a software protocol stack: SeaLinx. The testbed has the same design as the real system we deployed in sea. With the help of the lab testbed, we can conduct various tests in the lab including protocols evaluations and underwater application developments.