TDMA protocols have attracted a lot of attention for underwater acoustic sensor networks (UWSNs), because of the unique characteristics of acoustic signal propagation such as great energy consumption in Spindle Mixer Parts and Accessories transmission, long propagation delay and long communication range.Previous TDMA protocols all allocated transmission time to nodes based on discrete time slots.This paper proposes an efficient continuous time scheduling TDMA protocol (ECS) for UWSNs, including the continuous time based and sender oriented conflict analysis model, the transmission moment allocation algorithm and the distributed topology maintenance algorithm.Simulation results confirm that ECS improves network throughput by 20% on Swivel Chair (set of 2) average, compared to existing MAC protocols.