In the report a wireless sensor network for passive as well as active acoustic surveillance under water is described. A dedicated use is surveillance of an area at sea like the entrance to a harbour or a base. With other sensors the network also has a civilian use, e.g. for environmental surveillance. The network has a decentralised architecture and a large part of the signal processing is done locally within each node and a group of network nodes, after which the result is presented to the operations centre or similarly. Being autonomous and wireless the system is rapidly deployed, and covert if needed. The major part of the report deals with how to design of a smaller experimental network with reduced functionality. The computational demand for some simple surveillance algorithms shows that the needed computational power is low, far below what a modern PC can mange. The communication algorithms put higher computational requirements but can still be implemented. To obtain a low current concumption, thus longer battery life time, the serial produced nodes must be equipped with more specialised FFGA solutions for implementation of the different algorithms.