A multihop underwater acoustic network, which consists of a series of equal-distance hops connected by relay transceivers in a tandem, is considered. Messages are originated as coded packets from a source node at one end, relayed (decoded and re-encoded) sequentially hop by hop, and finally received by a destination node at the other end of the network. Several key characteristics of underwater acoustic channels, namely, frequency-dependent signal attenuation and noise, inter-hop interference, half-duplex constraint, and large propagation delay, are taken into account in the analysis. A simple transmission protocol with spatial reuse is considered, and the transmission schedule is designed to satisfy the half-duplex constraint on relay transceivers in the presence of large propagation delay. To efficiently cope with frequency-dependent channel characteristic and inter-hop interference, the power spectral density function of signaling is analytically optimized in a way analogous to water-filling. Furthermore, the problem of determining the minimum number of hops to support a prespecified rate and reliability with and without a maximum coded packet length constraint is examined. Finally, numerical results are presented to illustrate the analysis.