In spite of the body of work in modeling the underwater channel, there is still considerable lack of fundamental understanding that is required to enable the widespread deployment of Underwater Acoustic Networks (UANs). In this paper we study one such aspect, namely the impact of relay placement on the overall transmit power required to maintain a certain quality of communication between two nodes, in a multi-hop underwater line network. We start by analyzing a simple case involving one relay and a common signal frequency over all hops. We then extend the analysis to include different signal frequencies over different hops. Specifically, in each case, we find through analysis and numerical evaluation the optimal relay placement that minimizes total transmit power, thereby improving energy efficiency. We observe that the optimal relay locations are not always equidistant, especially when each hop can choose from a set of signal frequencies for transmission.