Long propagation delay, limited bandwidth, and high bit error rate pose great challenges in media access control (MAC) protocol design for underwater acoustic networks. A MAC protocol called slotted fl oor acquisition multiple access (slotted-FAMA) suitable for underwater acoustic networks is proposed and analyzed. This FAMA based protocol adds a time slot mechanism to avoid DATA packet collisions. However, slotted-FAMA is not suitable for dense networks since the multiple request-to-send (RTS) attempts problem in dense networks is serious and greatly limits the network throughput. To overcome this drawback, this paper proposes a slotted-FAMA based MAC protocol for underwater acoustic networks, called RC-SFAMA. RC-SFAMA introduces an RTS competition mechanism to keep the network from high frequency of backoff caused by the multiple RTS attempts problem. Via the RTS competition mechanism, useful data transmission can be completed successfully when the situation of multiple RTS attempts occurs. Simulation results show that RC-SFAMA increases the network throughput e?ciency as compared with slotted-FAMA, and minimizes the energy consumption.