This paper proposes a MAC protocol for enhancing the throughput in underwater acoustic sensor networks. Since the transmission speed of acoustic wave is much slower than that of radio, the propagation delay in the water is much longer. For solving the long propagation delay and hidden terminal problems, the throughput may become very low. To improve the throughput, this paper proposes an algorithm to increase the communications by using some resources which are originally idle for protecting communication from interference. The proposed protocol is based on synchronized technologies. Performance study reveals that the proposed protocol outperforms existing work in terms of network throughput and waiting time of nodes.