Abstract: Challenges of designing reliable and efficient Medium Access Control (MAC) protocols are posed for underwater acoustic sensor networks, because of hostile seawater channel condition. For hexagonal grid topology networks, this paper presents G-TDMA protocol, which improves channel utility with spatial reuse of time slots and reduces synchronic overhead by synchronizing nodes in the process of packet transmissions. In addition, G-TDMA employs guard time to avoid packet collisions due to long propagation delay and channel fluctuations. Simulation tests show that the protocol improves network performances in underwater environment.