Recent surges in the development of underwater acoustic networks (UANs) have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, limited work has been performed on developing secure communication mechanisms and techniques to protect these networks. Security mechanisms are wildly studied in terrestrial networks, and various defense mechanisms have been developed as safeguards. Because of the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UANs. In this paper, we study the effects of denial-of-service jamming attacks on UANs using real-world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on multiple commercial brand acoustic modems and an orthogonal frequency division multiplexing modem prototype. We show that UANs can be easily jammed using carefully timed attacks, which are energy efficient. Copyright © 2012 John Wiley & Sons, Ltd.