In this paper we discuss issues related to the design of underwater acoustic network protocols which are tailored around, and leverage on, the differences between underwater acoustics and terrestrial radio. These differences span physical propagation and energy consumption, and influence the design of medium access control, routing and topology management. By first reviewing a simple model for underwater sound propagation and hardware energy consumption, we introduce a set of solutions which explicitly account for, or make use of, the longer propagation delays of acoustic waves in the water with respect to radio waves in the air, and the different ratio between transmit and receive energy consumption offered by underwater transducers. These protocols deal with the problems of efficiently scheduling transmissions in a fixed 3D deployment, of optimizing the use of energy by choosing the best mechanisms for topology management, and of choosing the best hop length over a multihop path. We then review some more realistic underwater sound propagation behaviors, and detail their consequences on the simulation of MAC protocols for underwater networks. Finally, we briefly discuss the currently available underwater communications hardware (including both commercial and research modems) and comment on which paradigms are currently realizable, with special regard to those requiring the adaptability of transmit power and frequency.