Abstract: Underwater acoustic networks are characterized by long propagation delays and limited energy. DBR (Depth-Based Routing) protocol is main way to delivery packets in routing layer. In DBR, the packets are forwarded from deeper nodes to shallower nodes and the direction bias to the sink nodes. The bias of packets transmitting may lead to load unbalance in the UWSNs. However, the nodes in the current MAC protocols are normally fair which will not be sure the advantage position of the key nodes. If the key nodes cannot delivery enough packets that lead to network congestion. The network congestion will decline both energy and time effciency. In this paper, we propose a new Routing-Enhanced Depth-based MAC protocol called RD-MAC, which exploits cross-layer routing information in order to improve the energy and time effciency. Using ToA ranging technique, we can get the distance between sink nodes and underwater sensor nodes which can offer angle information between each node and sink node. This guarantee the superiority of the key nodes on the key route and significantly reduces both the propagation delay and congestion along each route. A better solution is found out to provide shorter end-to-end latency with single sink without sacrificing energy effciency, and cost for multi-sink maintenance is saved.