I was able to track boats off of the central Jersey coast by receiving the Automated Identification System (AIS) transponder signal. The signal is sent periodically and picked up by my VHF yagi antenna from as far as New York City about 60 miles away over open ocean.
The signal goes to an RTL-SDR running AiSDeco2 which demodulates the signal. AiSDeco2 also hosts a TCP server for OpenCPN navigational chart software to get the current locations and plot them on a map.
OpenCPN can be loaded with charts from NOAA to give you a more detailed map about the area you are working in. The craft locations are overlaid on the charts.
Crafts send the signal on two frequencies: 161.975 MHz and 162.025 MHz. AiSDeco2 with an RTL-SDR can handle both at the same time. If you watch the signal on a spectrogram waterfall chart, you will see an pretty easily noticed signature to help you aim the antenna.
In addition to crafts, you will also find that some navigational aids like buoys also transmit AIS.