Abstract: Medium access control (MAC) protocol is a key technology for underwater acoustic communication network (UACN). Unlike terrestrial wireless communication which uses radio waves, UACN relies on acoustic waves to communicate. The MAC protocol design of UACN faces many challenges: long propagation delay, low bandwidth, the battery energy cannot be replaced or recharge, the emission power limitation at nodes, etc. As a consequence, the MAC protocol of terrestrial radio networks cannot be applied directly in underwater acoustic networks. In this paper, we propose a new MACA-based MAC protocol called MACA-C with the combination of transmitting data packet and control packet. It improves the channel utilization by transmitting the first packet of packet train and RTS control packet during each round of handshake. The simulations show that the MACA-C is able to achieve high and stable throughput performance, increase channel utilization while maintaining low collision rate.