The first challenge of this project is development of a low cost hydrophone. Fortunately most of the legwork has already been done for me. Governments have funded research on vessel fundamental frequencies, weekend hackers have developed and tested several DIY piezio diaphragm hydrophone designs, and a couple researchers even developed a frequency response curve for a common DIY hydrophone build.
According to a paper titled "Acoustic surveillance of small surface vessels in confined areas" by A. Tesei et. al of the NATO Undersea Research Center in Italy, the fundamental frequency and source level of 7m outboard motor and fishing boats range from 0.25 to 1 kHz and 150 to 156 dB re 1uPa @ 1m respectively.
You can find dozens of DIY piezio diaphragm hydrophone builds online, but this one , do-it-yourself hydrophones, by John Grzinich in Istanbul even includes a sound recording of motor boats.
Over to the west side of the Atlantic, two researchers Eduardo Vivas and Braulio Lopez submitted a paper titled "Construction, calibration, and field test of a home-made, low-cost hydrophone for cetacean acoustic research." to the Acoustical Society of America. The paper details a frequency response curve of a DIY hydrophone. The frequency response curve starts from -55 dB re 1 V/uPa near 600Hz and continues upward to -25 db re 1 V/uPa at 8kHz. So capturing lower frequencies down to 250 Hz that are characteristic of fishing boats may be a challenge, but otherwise its still worth getting some parts on order and building one myself to test.