With the emphasis on effective safeguarding of the national marine rights and interests,the upsurge in marine economic development and the significant progress in wireless sensor networks, underwater sensor networks have become a new hot research area. Underwater sensor networks are deployed to perform collaborative tasks such as oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation, and tactical surveillance. Underwater sensor networks use acoustic signals to communicate with each other, thus the propagation delay is large due to the slow acoustic signal propagation speed. Underwater acoustic communication channel has limited bandwidth capacity because of the significant frequency and distance dependent attenuation. The harsh underwater channel also has high bit error rates. Thus, some new algorithms need designing for underwater sensor networks. An overview of underwater wireless communication technologies, the design of communication hardware, the underwater network architecture and the characteristics of underwater communication channels are introduced firstly. Then given is a more detailed description of eight important research topics: acoustic modulation, medium access control, routing protocols, transport protocols, cross-layer design, networks information processing, localization techniques and simulation environments. Research organizations on underwater sensor networks are also introduced. Finally presented are present problems encountered in current research and future research directions in this exciting and emerging research area.