Underwater acoustic networks have the potential to support a large variety of applications, such as environmental and equipment monitoring. However, underwater protocol design is in its infancy. Although there has been some work in routing and MAC layer protocols, they only address some of the challenges. A fundamental primitive that has not yet been researched for underwater networks is reliable broadcast. Reliable broadcast is required by many different applications, such as in-network node reprogramming. In this paper, we present three reliable broadcasting protocols (SBRB, FSBRB, and DBRB) that address the specific challenges of the underwater channel. We also compare our approach to two standard reliable broadcast protocols through extensive simulation, and show that our protocols provide significant gains in terms of both energy consumption and time to complete the broadcast. Moreover, our results demonstrate the importance of addressing the peculiar relationship between bandwidth and distance exhibited by an underwater acoustic channel.