In this paper, multiple access technique for the physical layer of underwater acoustic (UWA) networks is discussed. Considering the characteristics of acoustical channel and UWA networks, multi-carrier code division multiple access(MC-CDMA) method is chosen as the multiple access strategy in the design of an UWA network. On this basis, a MC-CDMA multiple access scheme is proposed. The preliminary results of simulation and lab pool testing indicate that MC-CDMA technique is a feasible multiple access method for the physical layer of UWA network.