Research in underwater acoustic networks has grown rapidly with the desire to monitor the large portion of the world covered by oceans. Fundamental differences between underwater acoustic propagation and terrestrial radio propagation call for new criteria for the design of networking protocols. In this paper, we focus on one of these fundamental differences, namely the dependence of usable bandwidth on transmission distance. The main contribution of this work is an in-depth analysis of the impacts of this unique relationship. Furthermore, the relationship between the energy consumptions of acoustic modems in various modes (i.e., transmit, receive, and idle) is different than that of their terrestrial radio counterparts, which also impacts the design of energy-efficient protocols. We present novel insights that are useful in guiding both protocol design and network deployment. We design an energy-efficient routing protocol for underwater sensor networks based on the insights gained in our analysis. This protocol is tested in a number of relevant network scenarios, and shown to significantly outperform greedy minimum link energy protocols, and to provide near optimal total path energy consumption. Finally, we implemented the underwater acoustic channel model in ns2 and used it to analyze the impact of multiple flows on our routing protocol's performance.