Opportunistic routing is suitable for Underwater Acoustic Sensor Networks (UASNs) with poor link quality and network stability, since it increases the probability and reliability of data transmission by coordinating multiple forwarding nodes. However, coordinating the data forwarding of multiple forwarding nodes introduces high energy consumption and end-to-end delay. The underwater acoustic directional transducer can concentrate the transmit and receive gains in a certain direction, reduce energy loss, and increase the single-hop transmission range, which is of great benefit to improving routing performance. Therefore, this paper proposes a directional transmission-based opportunistic routing protocol (DTOR) in UASNs. The combination of opportunistic routing and directional transmission is adopted to improve the reliability and timeliness of data transmission and reduce energy consumption. Simulation results show that the DTOR outperforms other routing protocols, e.g., GEDAR and WDFAD-DBR, in terms of packet delivery ratio, average end-to-end delay and energy consumption.