In this paper we study a number of design aspects for a large sensor field whose mission is to protect coastal waters by detecting objects like submarines. The system is buoy-based and distributed over a littoral area. The field has a number of gateway nodes that have the capability to transmit off-field through a satellite or an airplane. The system uses active sonar with acoustic sources sending acoustic energy (pings) and a number of sensor nodes receiving energy reflected by potential targets as well as non-target objects that may be present in the field, false alarms due to this clutter is controlled through proper fusion of sensor detections. In this paper, we discuss two important issues that need to be addressed when designing such sensor network: (1) Efficient in-network fusion of the sensor detections to improve the overall system-wide detection performance in terms of probability of detection and probability of false alarm. (2) Effects of gateway placements to the communications performance such as the average delay and packet drop rates. Simulation results are presented to characterize the trade-offs among different design choices.