Routing is one of the fundamental concerns in any communication network. Over the last two decades, a significant amount of attention has been given to the design of terrestrial routing protocols. However, the distinctive properties of the underwater communication channel - such as distance-dependent bandwidth, large propagation delay and time-varying mulitpath - render the existing terrestrial routing protocols implausible for underwater networks. In this paper we explore the challenges in developing a suitable routing protocol for underwater networks. We also look at existing routing protocols designed for both terrestrial and underwater networks, identify their shortcomings and finally lay out some guidelines for the design of routing protocols suitable for next generation underwater networks/applications.