We propose a location-free link state routing protocol for Underwater Acoustic Sensor Networks (UASNs). Additionally, we present the mathematical background for the theoretical capacity and transmission power metrics of an underwater acoustic channel. UASNs are formed by devices enabled with acoustic communication capabilities that are deployed underwater to perform collaborative monitoring tasks. Information is collected by a sink at the surface also equipped with a radio. The underwater communication channel is characterized by a limited bandwidth and high propagation delay. The network topology constantly changes due to mobility of the nodes. In our routing protocol, every node ranks the quality of the path that it offers toward the sink. Packet forwarding is performed hop-by-hop considering one or several routing metrics, e.g., hop count or pressure. To avoid communication void problems, every node selects a one-hop neighbor within an area that guarantees progress toward a sink. Our strategy is loop-free. It includes a recovery mode handling network topology changes. Our routing protocol was implemented in NS-3 to conduct experiments