Passive acoustic methods can detect, track and classify surface and underwater vessels. We consider the application of these systems for DHS interests with attention to small boats. We present a review of various methods of detection and tracking for low-cost low-power systems with a few hydrophones. The analysis of acoustic signatures of boats to determine their physical parameters (shaft and engine rate, number of propellers and blades, and engine firing rate) is discussed in detail. These parameters can be used for vessel classification. The analysis used acoustic recordings of boats from the Stevens acoustic library.